feat: Phase 3-4 Weeks 5+6 — off-grid Bitcoin ops + emergency alert system

Bitcoin relay (mesh/bitcoin_relay.rs):
- BlockHeaderCache: stores latest block headers from internet peers for SPV
- RelayTracker: tracks in-flight TX and Lightning relay requests
- Builder functions: block header announcements (Ed25519 signed),
  TX relay request/response, Lightning invoice relay/response
- All amounts as u64 sats, never float
- 4 unit tests

Emergency alerts (mesh/alerts.rs):
- AlertConfig: dead man switch settings, GPS, emergency contacts
- DeadManSwitch: background timer, auto-trigger after configurable interval
  (default 6h), signed alert broadcast with GPS coordinates
- check_in() resets timer, is_triggered() checks elapsed time
- GPS as integer microdegrees (Coordinate type from message_types)
- Disk persistence for config
- 4 unit tests

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/mesh/mod.rs

@@ -4,6 +4,10 @@
 //! Supports Meshcore firmware on Heltec V3, T-Beam, RAK WisBlock, Station G2,
 //! and other ESP32/nRF52-based LoRa boards via USB serial (Companion USB mode).
 
+#[allow(dead_code)]
+pub mod alerts;
+#[allow(dead_code)]
+pub mod bitcoin_relay;
 #[allow(dead_code)]
 pub mod crypto;
 #[allow(dead_code)]