feat: multi-sensor aggregation in companion protocol

[dougborg]

Summary

Support receiving and aggregating NDJSON streams from multiple AirHound sensors in the companion app. This enables coverage extension (multiple rooms/angles), RSSI triangulation, and more robust detection — without any firmware changes.

Inspired by the ESP32DualBandWardriver's Node/Core ESP-NOW mesh, but adapted to AirHound's companion-centric architecture. Sensors don't talk to each other; the companion app handles all aggregation.

Protocol Changes

• Add optional sensor_id field to DeviceMessage (protocol v2, spec: NDJSON wire protocol v1 schema and v2 roadmap #9)
• Each sensor identifies itself with a unique ID (board MAC, user-assigned name, or auto-generated)
• Companion app receives NDJSON streams from N sensors over any transport (BLE, serial, WebSocket Add WebSocket event streaming #83)

Companion-Side Design

• Stream multiplexing — companion maintains N concurrent NDJSON readers
• MAC deduplication — same device seen by multiple sensors is merged into one record
• RSSI merge strategy — options: max (closest sensor), weighted average, or per-sensor history
• Sensor health monitoring — detect disconnected/lagging sensors, alert on gaps

Deployment Patterns

• Room coverage — sensors in different rooms, companion sees whole building
• Perimeter monitoring — sensors at entry points, triangulate by which sensors see a device
• Mobile + fixed — carry one sensor, leave others stationary for baseline comparison
• Direction finding — with GPS-tagged sensors (feat: GPS integration and multi-sensor direction finding #15), RSSI from multiple angles enables coarse localization

Relates To

• spec: NDJSON wire protocol v1 schema and v2 roadmap #9 — protocol v2 (where sensor_id field gets added)
• feat: GPS integration and multi-sensor direction finding #15 — multi-sensor direction finding
• arch: multi-platform architecture — shared core, multiple runtimes #17 — multi-platform architecture
• Extend NDJSON protocol with new message types #71 — companion app
• Add WebSocket event streaming #83 — WebSocket transport