feat: initial meshservice implementation

Generic decentralized service layer for mesh networks.
Includes FAPP (psychotherapy) and Housing as reference services.

- ServiceMessage with Ed25519 signatures
- Wire protocol (64-byte header + CBOR)
- ServiceRouter with pluggable handlers
- Verification framework (3 trust levels)
- 32 tests, 3 examples

CLAUDE.md

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+# MeshService — Agent Notes
+
+## Purpose
+
+Generic decentralized service layer. Any P2P service following Announce→Query→Response→Reserve can be built on this.
+
+## Quick Commands
+
+```bash
+# Run tests
+cargo test
+
+# Run examples
+cargo run --example fapp_service
+cargo run --example housing_service
+cargo run --example multi_service
+```
+
+## Key Files
+
+| File | Purpose |
+|------|---------|
+| `src/router.rs` | Main entry point, `ServiceRouter` |
+| `src/message.rs` | `ServiceMessage` struct |
+| `src/services/fapp.rs` | FAPP implementation (reference) |
+| `src/services/housing.rs` | Housing (second service example) |
+
+## Adding a New Service
+
+1. Create `src/services/myservice.rs`
+2. Define payload structs with serde
+3. Implement `ServiceHandler`
+4. Add to `src/services/mod.rs`
+5. Add service ID to `lib.rs`
+
+## Wire Format
+
+64-byte header (fixed) + signature (64 bytes) + payload (CBOR) + optional verifications.
+
+See `src/wire.rs` for encoding/decoding.
+
+## Integration with quicprochat-p2p
+
+The `ServiceRouter` is transport-agnostic. To integrate:
+
+```rust
+// Receive from transport
+let msg = wire::decode(&incoming_bytes)?;
+let action = router.handle(msg, Some(sender_pubkey))?;
+
+// Handle action
+match action {
+    ServiceAction::StoreAndForward => {
+        // Re-broadcast msg.forwarded() via transport
+    }
+    ServiceAction::Respond(response) => {
+        // Send response back
+    }
+    _ => {}
+}
+```
+
+## Costs
+
+- No external API calls
+- Pure Rust, runs locally
+- Memory: Store grows with cached announces (configurable limits)