Feat/mesh robustness efficiency (#451)

* chat: de-dup private chats across ephemeral/stable IDs; prefer most advanced delivery status\n\n- Fixes LazyVStack duplicate ID warnings and blank row in PM\n- Merges messages by id from ephemeral and Noise-key stores\n- Chooses read > delivered > partiallyDelivered > sent > sending > failed (newer wins on tie)\n- Ensures status icon updates immediately without waiting for another send\n- Adds exhaustive handling for DeliveryStatus in ranking

* logging: reduce noisy info logs to debug; keep errors/warnings\n\n- Downgrade routing/ACK/subscription/connect logs to debug\n- Retain security/fingerprint/keychain info logs\n- Keep errors and warnings intact\n\ndocs: add docs/privacy-assessment.md covering BLE privacy, routing TTL/jitter, Nostr E2E gift wraps, ACK throttling, and logging posture

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Co-authored-by: jack <jackjackbits@users.noreply.github.com>

docs/privacy-assessment.md

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+BitChat Privacy Assessment
+==========================
+
+Scope
+- Mesh transport (BLE) behavior and metadata minimization
+- Nostr-based private message fallback (gift-wrapped, end-to-end encrypted)
+- Read receipts and delivery acknowledgments
+- Logging/telemetry posture and controls
+
+Summary
+- No accounts, no servers for mesh; Nostr used only for mutual favorites, with end-to-end Noise encryption encapsulated in gift wraps.
+- BLE announces contain only nickname and Noise pubkey. No device name, no plaintext identity beyond what the user broadcasts.
+- Discovery and flooding incorporate jitter and TTL caps to reduce linkability and propagation radius of encrypted payloads.
+- UI and storage remain ephemeral; message content is not persisted to disk. Minimal state (e.g., read-receipt IDs) is stored for UX and is bounded/cleaned.
+- Logging defaults to conservative levels; debug verbosity is suppressed for release builds. A single env var can raise/lower threshold when needed.
+
+BLE Privacy Considerations
+- Announce content: Unchanged — nickname + Noise public key only.
+- Local Name: Not used (explicitly disabled). Avoids leaking device/OS identity.
+- Address: iOS uses BLE MAC randomization; BitChat does not attempt to set static addresses.
+- Announce jitter: Each announce is delayed by a small random jitter to avoid synchronization-based correlation.
+- Scanning: Foreground scanning uses “allow duplicates” briefly to improve discovery latency; background uses standard scanning parameters.
+- RSSI gating: The acceptance threshold adapts to nearby density (approx. -95 to -80 dBm) to reduce long-distance observations in dense areas and improve connectivity in sparse ones.
+- Fragmentation: Fragments use write-with-response for reliability (less re-broadcast churn = fewer repeated signals).
+- GATT permissions: Private characteristic disallows .read; we use notify/write/writeWithoutResponse to avoid exposing plaintext attributes over GATT.
+
+Mesh Routing and Multi-hop Limits
+- Encrypted relays permitted with random per-hop delay (small jitter) to smooth floods.
+- TTL cap: Encrypted payloads are capped at 2 hops, limiting metadata spread and path reconstruction risk while enabling close-range relays.
+
+Nostr Private Messaging Fallback
+- Usage criteria: Only attempted for mutual favorites or where a Nostr key has been exchanged (stored in favorites).
+- Payload confidentiality: Messages embed a BitChat Noise-encrypted packet inside a NIP-17 gift wrap; relays see only random-looking ciphertext.
+- Timestamp handling: Gift wraps add small randomized offsets to reduce exact timing correlation.
+- Read/delivery acks: Also encapsulated in gift wraps, preserving content secrecy and minimizing metadata.
+- Relay policy variance: Some relays apply “web-of-trust” policies and may reject events; BitChat tolerates partial delivery and still prefers mesh when available.
+
+Read Receipts and Delivery Acks
+- Routing policy: Prefer mesh if Noise session established; otherwise use Nostr when mapping exists.
+- Throttling: Nostr READ acks are queued and rate-limited (~3/s) to prevent relay rate limits during backlogs.
+- Coalescing (optional future): When entering a chat with many unread, only send READ for the latest message, marking older as read locally to reduce metadata.
+
+Data Retention and State
+- Messages: Ephemeral in-memory only; history is bounded per chat and trimmed.
+- Read-receipt IDs: Stored in `UserDefaults` for UX continuity; periodically pruned to IDs present in memory.
+- Favorites: Noise and optional Nostr keys with petnames; can be wiped via panic action.
+- Panic: Triple-tap clears keys, sessions, cached state, and disconnects transports.
+
+Logging and Telemetry
+- Centralized `SecureLogger` filters potential secrets and uses OSLog privacy markers.
+- Default level: `info`; release builds suppress debug. Developers can set `BITCHAT_LOG_LEVEL=debug|info|warning|error|fault`.
+- Transport routing, ACK sends, subscribe/connect noise were downgraded from info→debug.
+- OS/system errors (e.g., transient WebSocket disconnects) may still appear in system logs; BitChat avoids re-logging those unless actionable.
+
+Residual Risks and Mitigations
+- RF fingerprinting: BLE presence is observable at the RF layer; mitigated by minimal announce content and platform MAC randomization.
+- Timing correlation: Announce/relay jitter reduces but does not eliminate timing analysis. Avoids synchronized bursts.
+- Relay metadata: Nostr relays can see that an account posts gift wraps; content remains end-to-end encrypted. Favor mesh path when in range.
+
+Recommendations (Next)
+- Add optional coalesced READ behavior for large backlogs.
+- Expose a “low-visibility mode” to reduce scanning aggressiveness in sensitive contexts.
+- Allow user-configurable Nostr relay set with a “private relays only” toggle.
+