feat(fapp): add FappRouter for mesh integration

New fapp_router.rs module:
- FappAction enum (Ignore, Dropped, Forward, QueryResponse)
- Wire format: 1-byte tag (0x01-0x05) + CBOR body
- FappRouter with shared RoutingTable and TransportManager
- handle_incoming() decodes and dispatches FAPP frames
- process_slot_announce() with relay/flood logic
- process_slot_query() answers from local FappStore
- broadcast_announce() / send_query() for outbound floods
- drain_pending_sends() for async send integration
- 3 unit tests

Also fixed borrow checker issue in FappStore::store

crates/quicprochat-p2p/src/lib.rs

@@ -16,6 +16,7 @@ pub mod address;
 pub mod announce;
 pub mod announce_protocol;
 pub mod fapp;
+pub mod fapp_router;
 pub mod broadcast;
 pub mod envelope;
 pub mod envelope_v2;