refactor: extract the shared rolling-baseline core

BleFieldCore and MagFieldCore had converged on the same class: same
time-aware EMA, same warm-up gate, same "deviation past a threshold is an
event" shape, differing only in constants and field names. Three copies of
the alpha derivation existed across the sensor libs.

lib/baseline.ts now owns it. Both wrappers keep their own public types
(`.rssi`, `.magnitude`) so nothing downstream changed — which is what let
all 26 existing tests pass completely unmodified against the refactor.
That was the point of doing it this way: if the tests had needed editing,
the refactor would have been changing behaviour rather than removing
duplication.

coldSpot.ts deliberately does NOT adopt this. It threads immutable state
through pure functions so a whole session's narrative can be replayed in a
test without a clock — a different and equally valid shape. Collapsing the
two would force one into a style that doesn't fit it, which is how
deduplication turns into damage.

355 frontend tests pass unchanged; i18n parity gate passes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/lib/bluetooth.ts

@@ -46,6 +46,8 @@ function bluetoothApi(): BluetoothLike | null {
   return nav.bluetooth ?? null
 }
 
+import { ThresholdBaseline } from './baseline'
+
 export type BleReading = {
   /** Received signal strength, dBm. Typically -30 (touching) to -100 (far). */
   rssi: number
@@ -96,55 +98,36 @@ export function isSupported(): boolean {
  * Pure and separately testable: no Bluetooth objects appear in here.
  */
 export class BleFieldCore {
-  private mean: number | null = null
-  private lastAt: number | null = null
-  private samples = 0
+  private readonly core = new ThresholdBaseline({
+    threshold: DISTURBANCE_DB,
+    tauMs: BASELINE_TAU_MS,
+    minSamples: MIN_SAMPLES,
+  })
 
   get baseline(): number | null {
-    return this.mean
+    return this.core.baseline
   }
 
   get warm(): boolean {
-    return this.samples >= MIN_SAMPLES
+    return this.core.warm
   }
 
-  /**
-   * Fold one reading in. Returns a disturbance when the deviation clears
-   * the threshold and the baseline is warm, else null. Non-finite input is
-   * a no-op rather than a crash or a poisoned baseline.
-   */
+  /** Fold one reading in; see ThresholdBaseline.push. Returns a
+   * BLE-flavoured disturbance so callers keep reading `.rssi` rather than a
+   * generic `.value`. */
   push(rssi: number, atMs: number): BleDisturbance | null {
-    if (!Number.isFinite(rssi) || !Number.isFinite(atMs)) return null
-
-    if (this.mean === null) {
-      this.mean = rssi
-      this.lastAt = atMs
-      this.samples = 1
-      return null
-    }
-
-    const deviation = rssi - this.mean
-    const warm = this.warm
-
-    const dt = this.lastAt === null ? 0 : atMs - this.lastAt
-    const alpha = dt > 0 ? 1 - Math.exp(-dt / BASELINE_TAU_MS) : 0.15
-    this.mean = this.mean + alpha * deviation
-    this.lastAt = atMs
-    this.samples++
-
-    if (!warm || Math.abs(deviation) < DISTURBANCE_DB) return null
+    const event = this.core.push(rssi, atMs)
+    if (!event) return null
     return {
-      deviation,
-      severity: Math.min(1, Math.abs(deviation) / (DISTURBANCE_DB * 3)),
-      rssi,
-      at: atMs,
+      deviation: event.deviation,
+      severity: event.severity,
+      rssi: event.value,
+      at: event.at,
     }
   }
 
   reset(): void {
-    this.mean = null
-    this.lastAt = null
-    this.samples = 0
+    this.core.reset()
   }
 }