fix: Phase 8 — mesh hardening: atomic writes, unwrap elimination, GPS opt-out

- Ratchet state: atomic write via tmp + rename to prevent corruption on crash
- Block header decode: replaced .unwrap() with proper error handling on
  untrusted network data (was a crash vector from malicious peers)
- Shutdown channel: replaced .unwrap() with .ok_or_else() error propagation
- Dead man's switch GPS: default changed to opt-out (auto_include_gps=false)
- Alert signature verification: already covered by Phase 4 envelope checks

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

core/archipelago/src/mesh/alerts.rs

@@ -51,7 +51,7 @@ impl Default for AlertConfig {
             dead_man_interval_secs: DEFAULT_INTERVAL_SECS,
             last_gps: None,
             emergency_contacts: Vec::new(),
-            auto_include_gps: true,
+            auto_include_gps: false,
             custom_message: None,
         }
     }