feat: Phase 3 Week 4 — mesh RPC endpoints for typed messages + session management

Backend (6 new RPC endpoints):
- mesh.send-invoice: create Lightning invoice, send bolt11 to mesh peer
- mesh.send-coordinate: send GPS coordinates (integer microdegrees)
- mesh.send-alert: send signed emergency alert (with optional GPS)
- mesh.outbox: list pending store-and-forward messages
- mesh.session-status: get Double Ratchet session info per peer
- mesh.rotate-prekeys: force X3DH prekey rotation

Mock backend: matching dev mode responses for all 6 new endpoints

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

core/archipelago/src/api/rpc/mod.rs

@@ -646,6 +646,12 @@ impl RpcHandler {
             "mesh.send" => self.handle_mesh_send(params).await,
             "mesh.broadcast" => self.handle_mesh_broadcast().await,
             "mesh.configure" => self.handle_mesh_configure(params).await,
+            "mesh.send-invoice" => self.handle_mesh_send_invoice(params).await,
+            "mesh.send-coordinate" => self.handle_mesh_send_coordinate(params).await,
+            "mesh.send-alert" => self.handle_mesh_send_alert(params).await,
+            "mesh.outbox" => self.handle_mesh_outbox(params).await,
+            "mesh.session-status" => self.handle_mesh_session_status(params).await,
+            "mesh.rotate-prekeys" => self.handle_mesh_rotate_prekeys().await,
 
             // Transport layer (unified routing)
             "transport.status" => self.handle_transport_status().await,