fix: address P0/P1/P2 audit findings — security, reliability, CI, docs

P0 - Critical:
- config.py: add _mask() credential-redaction helper, SETTINGS_SCHEMA_VERSION,
  plaintext-storage warning; corrupt settings.json backed up before defaults
- gost_util.py: pin SHA256 for gost_3.2.6_windows_amd64.zip; verify before
  extraction; _add_defender_exclusion now logs warning on failure
- firewall.py: add emergency_disengage() for atexit/signal use
- service.py: register fw_emergency_disengage via atexit + SIGTERM/SIGINT;
  stop() calls emergency_disengage if thread hangs past 15s timeout
- app.py: wrap main() in top-level except with CTk error dialog + log
- LICENSE: add MIT license file

P1 - Important:
- app.py: switch to RotatingFileHandler (5 MB / 3 backups)
- config.py: settings_version + migrate(); save_settings() writes .bak before
  overwrite; _is_safe_https_url() strips RFC-1918 sources/ip_check_url
- service.py: threading.Lock on _settings; _signal_handler; graceful stop
- requirements.txt: pin exact versions; add cryptography==48.0.0
- .gitignore: add settings.json, credential JSON files, screenshot noise
- tray.py, dns_leak.py, gost_util.py: replace bare except:pass with logging
- CHANGELOG.md: document all session changes

P2 - Nice to have:
- .github/workflows/test.yml: CI on Python 3.10/3.11/3.12 windows-latest
- run.py: --version / -V flag
- docs/OPERATOR_RUNBOOK.md: emergency disengage, proxy leak, GOST, settings

Tests: 47/47 passed (python -m unittest discover -s tests -v)
Co-authored-by: Cursor <cursoragent@cursor.com>

proxy_chain_manager/firewall.py

@@ -160,3 +160,18 @@ def is_engaged() -> bool:
     """Quick check: are our rules present?"""
     r = _run(["netsh", "advfirewall", "firewall", "show", "rule", f"name={RULE_PREFIX}GOST", "dir=out"])
     return RULE_PREFIX in (r.stdout or "")
+
+
+def emergency_disengage() -> None:
+    """Best-effort kill-switch removal for atexit/signal handlers.
+
+    Unlike ``disengage()``, this never raises and does not require prior
+    admin check — it simply tries, logs the outcome, and returns.  Safe to
+    call from atexit or signal handlers where exceptions must not propagate.
+    """
+    try:
+        _set_outbound_policy("blockinbound,allowoutbound")
+        _delete_rules()
+        log.info("emergency_disengage: firewall rules removed.")
+    except Exception as exc:  # noqa: BLE001
+        log.warning("emergency_disengage failed: %s", exc)