feat: possession presentation layer — glitchy text + degraded audio

First sub-project of the "make contact feel real" arc (spec:
docs/superpowers/specs/2026-07-23-possession-presentation-design.md).
Direct Contact replies now feel like a spirit fighting through static to
hold the channel rather than a plain chat bubble:

- backend/app/possession.py: compute_stability(rarity, magnitude, rng) —
  a 0.05-0.98 score per reply (rarer entity + stronger triggering anomaly =
  cleaner signal), rng injectable for a later quantum-RNG source.
- ws.py sends stability on reply_start; audio synthesis for that reply gets
  noise/bitcrush scaled by instability (1 - stability) via a new
  instability param on synthesize_spirit_voice — effects.py itself is
  untouched, only the params fed into it.
- frontend/src/lib/possession.ts: renderPossessedText — pure, deterministic
  (tick-seeded, no Math.random) text corruption with self-correcting
  glitch bursts, wired into Transcript.tsx's streaming reply display.

Stored transcript/reply text is unaffected — this is presentation only.
78/78 backend, 137/137 frontend tests passing.

backend/app/possession.py

@@ -0,0 +1,39 @@
+"""Possession stability: a single number expressing how cleanly a spirit
+holds the channel for one reply. Shared contract between the backend audio
+degradation and the frontend text-glitch renderer (see
+docs/superpowers/specs/2026-07-23-possession-presentation-design.md)."""
+
+import random
+from typing import Callable
+
+_BASE_BY_RARITY = {
+    "common": 0.35,
+    "uncommon": 0.5,
+    "rare": 0.65,
+    "mythic": 0.8,
+}
+
+_MIN_STABILITY = 0.05
+_MAX_STABILITY = 0.98
+_MAX_MAGNITUDE_BONUS = 0.15
+_JITTER_SPAN = 0.1  # rng() in [0, 1) is scaled to +/- this amount
+
+
+def compute_stability(
+    rarity: str,
+    magnitude: float,
+    rng: Callable[[], float] = random.random,
+) -> float:
+    """How cleanly the spirit holds the channel for this reply, 0.05-0.98.
+
+    Rarer spirits hold the channel more steadily (higher base). A stronger
+    triggering anomaly gives a cleaner line, up to a capped bonus. A final
+    rng-driven jitter of +/- 0.1 keeps it from feeling deterministic to the
+    seeker. `rng` is injectable so a later quantum-RNG source can swap in
+    real entropy without touching call sites.
+    """
+    base = _BASE_BY_RARITY.get(rarity, _BASE_BY_RARITY["common"])
+    magnitude_bonus = min(_MAX_MAGNITUDE_BONUS, magnitude / 100)
+    jitter = (rng() * 2 - 1) * _JITTER_SPAN
+    stability = base + magnitude_bonus + jitter
+    return max(_MIN_STABILITY, min(_MAX_STABILITY, stability))

backend/app/tts/piper.py

@@ -50,18 +50,48 @@ class PiperTTS:
 
 
 async def synthesize_spirit_voice(
-    text: str, voice: Voice, voice_profile: dict | None = None
+    text: str,
+    voice: Voice,
+    voice_profile: dict | None = None,
+    instability: float = 0.0,
 ) -> bytes:
-    """One call: Piper synth + the spirit's signature effects chain."""
+    """One call: Piper synth + the spirit's signature effects chain.
+
+    `instability` (0.0-1.0, i.e. `1 - stability`) scales up the noise and
+    bitcrush fed into the effects chain when the spirit is fighting to hold
+    the channel — a shaky possession sounds shakier. It never touches
+    `apply_effects` itself, only the values handed to it here.
+    """
     profile = voice_profile or {}
+    instability = max(0.0, min(1.0, instability))
     model_path = Path(settings.piper_voices_dir) / voice.model_file
+
+    base_noise = float(profile.get("noise", 0.03))
+    # Noise ramps linearly across the whole range so degradation is audible
+    # from the first sign of instability, but doubling the base level at
+    # instability=1.0 (rather than, say, 5x) keeps the words intelligible
+    # even at a full-static possession — this is texture, not white noise.
+    noise_level = base_noise + instability * base_noise
+
+    base_bitcrush = int(profile.get("bitcrush", 0))
+    # Bitcrush only kicks in once instability crosses ~0.5 -- a slightly
+    # unstable channel just sounds noisier, not crunchy/robotic. Above that
+    # threshold it ramps up to +6 bits of crush on top of whatever the
+    # voice's own profile already applies, capped at 15 (apply_effects only
+    # crushes when 0 < bits < 16).
+    if instability > 0.5:
+        bitcrush_bonus = round((instability - 0.5) / 0.5 * 6)
+    else:
+        bitcrush_bonus = 0
+    bitcrush_bits = min(15, base_bitcrush + bitcrush_bonus)
+
     wav = await PiperTTS(str(model_path)).synthesize(text)
     return await asyncio.to_thread(
         apply_effects,
         wav,
-        noise_level=float(profile.get("noise", 0.03)),
+        noise_level=noise_level,
         pitch_semitones=float(profile.get("pitch", 0.0)),
         rate=float(profile.get("rate", 1.0)),
-        bitcrush_bits=int(profile.get("bitcrush", 0)),
+        bitcrush_bits=bitcrush_bits,
         echo=float(profile.get("echo", 0.2)),
     )

backend/app/ws.py

@@ -36,6 +36,7 @@ from app.models.contact_session import ContactSession
 from app.models.entity import Entity
 from app.models.entity_sighting import EntitySighting
 from app.models.event import Event
+from app.possession import compute_stability
 from app.rate_limit import RateLimiter, resolve_client_ip
 from app.telemetry import detect_wire_spike, sample_network
 from app.tts.piper import synthesize_spirit_voice
@@ -147,7 +148,9 @@ async def _record_event(
         return event.id
 
 
-async def _speak(state: SeanceState, kind: str, text: str) -> None:
+async def _speak(
+    state: SeanceState, kind: str, text: str, instability: float = 0.0
+) -> None:
     """Persist an utterance, push its text immediately, synthesize audio in
     the background, and push the audio URL when the effects chain finishes."""
     event_id = await _record_event(
@@ -167,7 +170,9 @@ async def _speak(state: SeanceState, kind: str, text: str) -> None:
         try:
             voice_profile = state.entity.get("voice", {}) if state.entity else {}
             voice = pick_voice(voice_profile.get("voice_id"), state.language)
-            wav = await synthesize_spirit_voice(text, voice, voice_profile)
+            wav = await synthesize_spirit_voice(
+                text, voice, voice_profile, instability=instability
+            )
             filename = f"{event_id}.wav"
             _audio_dir().joinpath(filename).write_bytes(wav)
             async with session_maker() as db:
@@ -323,7 +328,12 @@ async def _handle_question(state: SeanceState, text: str) -> None:
     text = text.strip()[:500]
     await _record_event(state.session_id, "question", text=text)
     await state.send_queue.put({"type": "status", "state": "gathering"})
-    await state.send_queue.put({"type": "reply_start"})
+
+    last_magnitude = state.anomalies[-1].get("magnitude") if state.anomalies else 50
+    if last_magnitude is None:
+        last_magnitude = 50
+    stability = compute_stability(state.entity["rarity"], float(last_magnitude))
+    await state.send_queue.put({"type": "reply_start", "stability": stability})
 
     reply_parts: list[str] = []
     try:
@@ -350,7 +360,7 @@ async def _handle_question(state: SeanceState, text: str) -> None:
     state.history = state.history[-8:]
     await state.send_queue.put({"type": "reply_end", "id": str(reply_id), "text": reply})
     if reply:
-        await _speak(state, "reply", reply)
+        await _speak(state, "reply", reply, instability=1 - stability)
 
 
 async def _ambient_loop(state: SeanceState) -> None:

backend/tests/test_possession.py

@@ -0,0 +1,69 @@
+from app.possession import compute_stability
+
+
+def test_rarity_ordering_holds_magnitude_and_rng_constant():
+    # Midpoint rng (0.5) zeroes out jitter, isolating the rarity base.
+    rng = lambda: 0.5
+    common = compute_stability("common", magnitude=50, rng=rng)
+    uncommon = compute_stability("uncommon", magnitude=50, rng=rng)
+    rare = compute_stability("rare", magnitude=50, rng=rng)
+    mythic = compute_stability("mythic", magnitude=50, rng=rng)
+
+    assert common < uncommon < rare < mythic
+
+
+def test_unknown_rarity_falls_back_to_common_base():
+    rng = lambda: 0.5
+    assert compute_stability("legendary???", magnitude=50, rng=rng) == compute_stability(
+        "common", magnitude=50, rng=rng
+    )
+
+
+def test_magnitude_bonus_increases_stability_up_to_cap():
+    rng = lambda: 0.5
+    low = compute_stability("common", magnitude=0, rng=rng)
+    mid = compute_stability("common", magnitude=10, rng=rng)
+    high = compute_stability("common", magnitude=1000, rng=rng)
+
+    assert low < mid < high
+    # Bonus caps at 0.15, so magnitude=15 and magnitude=1000 must land the
+    # same once base and jitter are held constant.
+    at_cap = compute_stability("common", magnitude=15, rng=rng)
+    past_cap = compute_stability("common", magnitude=1000, rng=rng)
+    assert at_cap == past_cap
+
+
+def test_jitter_scaled_to_plus_minus_point_one():
+    # rng()=0 -> minimum jitter (-0.1); rng()=1 -> maximum jitter (+0.1).
+    base = 0.35  # common base
+    low_jitter = compute_stability("common", magnitude=0, rng=lambda: 0.0)
+    high_jitter = compute_stability("common", magnitude=0, rng=lambda: 1.0)
+
+    assert low_jitter == max(0.05, base - 0.1)
+    assert high_jitter == min(0.98, base + 0.1)
+
+
+def test_clamped_at_lower_bound():
+    # common base 0.35 minus full jitter (0.1) minus nothing else is 0.25,
+    # comfortably above the floor -- force the floor with a negative-leaning
+    # setup instead: rarity base + jitter alone can't go below 0.05, but the
+    # clamp itself must still be exercised directly via a pathological rng.
+    stability = compute_stability("common", magnitude=0, rng=lambda: -100.0)
+    assert stability == 0.05
+
+
+def test_clamped_at_upper_bound():
+    stability = compute_stability("mythic", magnitude=1000, rng=lambda: 100.0)
+    assert stability == 0.98
+
+
+def test_deterministic_with_injected_rng():
+    rng = lambda: 0.73
+    first = compute_stability("rare", magnitude=42, rng=rng)
+    second = compute_stability("rare", magnitude=42, rng=rng)
+    assert first == second
+
+
+def test_default_rng_produces_value_in_range():
+    stability = compute_stability("uncommon", magnitude=25)
+    assert 0.05 <= stability <= 0.98

backend/tests/test_tts_piper.py

@@ -0,0 +1,118 @@
+import pytest
+
+import app.tts.piper as piper
+from app.tts.voices import VOICES
+
+
+@pytest.fixture(autouse=True)
+def _fake_piper_synth(monkeypatch):
+    async def _fake_synthesize(self, text):
+        return b"fake-wav-bytes"
+
+    monkeypatch.setattr(piper.PiperTTS, "synthesize", _fake_synthesize)
+
+
+def _capture_effects_call(monkeypatch):
+    calls = []
+
+    def _fake_apply_effects(wav_bytes, **kwargs):
+        calls.append(kwargs)
+        return wav_bytes
+
+    monkeypatch.setattr(piper, "apply_effects", _fake_apply_effects)
+    return calls
+
+
+@pytest.mark.asyncio
+async def test_zero_instability_keeps_profile_defaults(monkeypatch):
+    calls = _capture_effects_call(monkeypatch)
+    voice = VOICES["lessac"]
+
+    await piper.synthesize_spirit_voice("hello", voice, {"noise": 0.03, "bitcrush": 0})
+
+    assert calls[0]["noise_level"] == pytest.approx(0.03)
+    assert calls[0]["bitcrush_bits"] == 0
+
+
+@pytest.mark.asyncio
+async def test_instability_increases_noise_level(monkeypatch):
+    calls = _capture_effects_call(monkeypatch)
+    voice = VOICES["lessac"]
+
+    await piper.synthesize_spirit_voice(
+        "hello", voice, {"noise": 0.03, "bitcrush": 0}, instability=1.0
+    )
+
+    # Meaningfully louder than the clean baseline, but not blown out.
+    assert calls[0]["noise_level"] > 0.03
+    assert calls[0]["noise_level"] <= 0.1
+
+
+@pytest.mark.asyncio
+async def test_noise_level_scales_monotonically_with_instability(monkeypatch):
+    voice = VOICES["lessac"]
+    noise_levels = []
+    for instability in (0.0, 0.25, 0.5, 0.75, 1.0):
+        calls = _capture_effects_call(monkeypatch)
+        await piper.synthesize_spirit_voice(
+            "hello", voice, {"noise": 0.03, "bitcrush": 0}, instability=instability
+        )
+        noise_levels.append(calls[0]["noise_level"])
+
+    assert noise_levels == sorted(noise_levels)
+    assert noise_levels[0] < noise_levels[-1]
+
+
+@pytest.mark.asyncio
+async def test_bitcrush_stays_off_below_threshold(monkeypatch):
+    voice = VOICES["lessac"]
+    for instability in (0.0, 0.2, 0.5):
+        calls = _capture_effects_call(monkeypatch)
+        await piper.synthesize_spirit_voice(
+            "hello", voice, {"noise": 0.03, "bitcrush": 0}, instability=instability
+        )
+        assert calls[0]["bitcrush_bits"] == 0
+
+
+@pytest.mark.asyncio
+async def test_bitcrush_kicks_in_above_threshold(monkeypatch):
+    calls = _capture_effects_call(monkeypatch)
+    voice = VOICES["lessac"]
+
+    await piper.synthesize_spirit_voice(
+        "hello", voice, {"noise": 0.03, "bitcrush": 0}, instability=1.0
+    )
+
+    assert calls[0]["bitcrush_bits"] > 0
+    assert calls[0]["bitcrush_bits"] < 16
+
+
+@pytest.mark.asyncio
+async def test_instability_is_clamped_to_unit_range(monkeypatch):
+    calls = _capture_effects_call(monkeypatch)
+    voice = VOICES["lessac"]
+
+    await piper.synthesize_spirit_voice(
+        "hello", voice, {"noise": 0.03, "bitcrush": 0}, instability=5.0
+    )
+    clamped_high = calls[0]
+
+    calls = _capture_effects_call(monkeypatch)
+    await piper.synthesize_spirit_voice(
+        "hello", voice, {"noise": 0.03, "bitcrush": 0}, instability=1.0
+    )
+    at_one = calls[0]
+
+    assert clamped_high == at_one
+
+
+@pytest.mark.asyncio
+async def test_instability_adds_on_top_of_profile_bitcrush(monkeypatch):
+    calls = _capture_effects_call(monkeypatch)
+    voice = VOICES["lessac"]
+
+    await piper.synthesize_spirit_voice(
+        "hello", voice, {"noise": 0.03, "bitcrush": 4}, instability=1.0
+    )
+
+    assert calls[0]["bitcrush_bits"] > 4

backend/tests/test_ws_session.py

@@ -31,7 +31,7 @@ class FakeSpiritService:
         return False
 
 
-async def _fake_synth(text, voice, profile):
+async def _fake_synth(text, voice, profile, instability=0.0):
     return b"RIFFfake wav bytes"
 
 
@@ -124,7 +124,8 @@ async def test_question_streams_reply_and_records_history(sync_client, db_sessio
         _read_until(ws, "session")
         ws.send_json({"type": "question", "text": "Are you at peace?"})
         _read_until(ws, "entity")  # auto-summoned before answering
-        _read_until(ws, "reply_start")
+        reply_start = _read_until(ws, "reply_start")
+        assert 0.05 <= reply_start["stability"] <= 0.98
         reply_end = _read_until(ws, "reply_end")
         assert reply_end["text"] == "I am here."