feat(p2p): add MeshNode integrating all production modules

New mesh_node.rs providing a production-ready node:
- MeshNodeBuilder for fluent configuration
- MeshConfig integration for all settings
- MeshMetrics tracking for all operations
- Rate limiting on incoming messages
- Backpressure controller
- Graceful shutdown via ShutdownCoordinator
- Optional FappRouter based on capabilities
- MeshRouter for envelope routing
- TransportManager for multi-transport support

Key APIs:
- MeshNodeBuilder::new().fapp_relay().build()
- node.process_incoming() with rate limiting + metrics
- node.gc() for store/routing table cleanup
- node.shutdown() for graceful termination

222 tests passing (203 lib + 3 fapp_flow + 16 multi_node)

crates/meshservice/examples/fapp_service.rs

@@ -0,0 +1,86 @@
+//! FAPP Service Demo
+//!
+//! Demonstrates therapist announcement and patient query flow.
+
+use meshservice::{
+    capabilities,
+    identity::ServiceIdentity,
+    router::ServiceRouter,
+    services::fapp::{create_announce, create_query, FappService, Modality, SlotAnnounce, SlotQuery, Specialism},
+};
+
+fn main() {
+    println!("=== FAPP Service Demo ===\n");
+
+    // Create identities
+    let therapist = ServiceIdentity::generate();
+    let patient = ServiceIdentity::generate();
+    let relay = ServiceIdentity::generate();
+
+    println!("Therapist address: {:?}", hex(&therapist.address()));
+    println!("Patient address:   {:?}", hex(&patient.address()));
+    println!("Relay address:     {:?}\n", hex(&relay.address()));
+
+    // Create router with FAPP service
+    let mut router = ServiceRouter::new(capabilities::RELAY);
+    router.register(Box::new(FappService::relay()));
+
+    // Therapist creates announcement
+    let announce = SlotAnnounce::new(
+        &[Specialism::CognitiveBehavioral, Specialism::TraumaFocused],
+        Modality::VideoCall,
+        "104", // Berlin Kreuzberg
+    )
+    .with_slots(3)
+    .with_profile("https://therapists.de/dr-schmidt")
+    .with_name("Dr. Anna Schmidt");
+
+    println!("Therapist announces:");
+    println!("  Specialisms: CBT, Trauma");
+    println!("  Modality: Video");
+    println!("  Location: 104xx");
+    println!("  Slots: 3");
+    println!("  Profile: https://therapists.de/dr-schmidt\n");
+
+    let msg = create_announce(&therapist, &announce, 1).unwrap();
+    let action = router.handle(msg.clone(), Some(therapist.public_key())).unwrap();
+    println!("Router action: {:?}", action);
+    println!("Stored messages: {}\n", router.store().len());
+
+    // Patient creates query
+    let query = SlotQuery::new(Specialism::CognitiveBehavioral, "104")
+        .with_modality(Modality::VideoCall)
+        .with_max_wait(30);
+
+    println!("Patient queries:");
+    println!("  Looking for: CBT");
+    println!("  Location: 104xx");
+    println!("  Modality: Video");
+    println!("  Max wait: 30 days\n");
+
+    let query_msg = create_query(&patient, &query).unwrap();
+
+    // Find matches
+    let matches = router.query(&query_msg);
+    println!("Found {} matching therapist(s):", matches.len());
+
+    for (i, m) in matches.iter().enumerate() {
+        if let Ok(data) = meshservice::services::fapp::SlotAnnounce::from_bytes(&m.message.payload) {
+            println!("  {}. {} in {}xx ({} slots)", 
+                i + 1,
+                data.display_name.as_deref().unwrap_or("Unknown"),
+                data.postal_prefix,
+                data.available_slots
+            );
+            if let Some(profile) = &data.profile_url {
+                println!("     Verify: {}", profile);
+            }
+        }
+    }
+
+    println!("\n=== Demo Complete ===");
+}
+
+fn hex(bytes: &[u8]) -> String {
+    bytes.iter().map(|b| format!("{b:02x}")).collect()
+}