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/session.rs

@@ -64,12 +64,17 @@ impl SessionManager {
             .await
             .context("Failed to create ratchet directory")?;
         let path = self.session_path(did);
+        let tmp_path = path.with_extension("tmp");
         let content = serde_json::to_string_pretty(state)
             .context("Failed to serialize ratchet session")?;
-        tokio::fs::write(&path, content)
+        // Atomic write: write to temp file, then rename
+        tokio::fs::write(&tmp_path, content)
             .await
-            .context("Failed to write ratchet session")?;
-        debug!(did = %did, "Saved ratchet session to disk");
+            .context("Failed to write temporary ratchet state")?;
+        tokio::fs::rename(&tmp_path, &path)
+            .await
+            .context("Failed to atomically rename ratchet state file")?;
+        debug!(did = %did, "Saved ratchet session to disk (atomic)");
         Ok(())
     }