fix: Firestore rules hardening, recovery phrase strength, test cleanup (batch v0.2.12)
- Firestore rules: add isCouplesMember(coupleId) to question thread answer writes (prevents outsider writes)
- Firestore rules: allow currentIndex increment on same-status session updates (fixes thread progression)
- RecoveryKeyManager: PHRASE_WORD_COUNT 6→10 (~80 bits entropy)
- build.gradle.kts: exclude META-INF/versions/9/OSGI-INF/MANIFEST.MF (packaging conflict)
- .gitignore: add firebase-debug.log, firestore-debug.log
- firestore-tests: configurable emulator port via FIRESTORE_EMULATOR_PORT env var
- firestore-tests: fix invite outsider test (seed with different coupleId), fix non-starter session test (active→completed allowed), remove redundant beforeEach(seedThread), add outsider-write-denied test for thread answers
- visual-identity.md: update encryption claim gating note
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