Implement spanning tree announcement send/receive protocol

Add TreeAnnounce v1 wire format with versioned encoding (version byte
0x01), slim ancestry entries (32 bytes each, no per-entry signatures),
and transitive trust model where only the direct peer's declaration
signature is verified.

Key changes:
- Enrich TreeCoordinate with CoordEntry metadata (sequence, timestamp)
- TreeAnnounce encode/decode with roundtrip tests
- Per-peer rate limiting (500ms minimum interval) on ActivePeer
- Parent selection: depth-based algorithm with broken-path detection
- New node/tree.rs module: send/receive, periodic refresh, cleanup
- Wire up dispatch, initial announce on promotion, tick integration
- Update gossip protocol design doc with trust model (§2.7)

304 tests pass, clean build.

src/lib.rs

@@ -29,7 +29,7 @@ pub use identity::{
 pub use config::{Config, ConfigError, IdentityConfig, TunConfig, UdpConfig};
 
 // Re-export tree types
-pub use tree::{ParentDeclaration, TreeCoordinate, TreeError, TreeState};
+pub use tree::{CoordEntry, ParentDeclaration, TreeCoordinate, TreeError, TreeState};
 
 // Re-export bloom filter types
 pub use bloom::{BloomError, BloomFilter, BloomState};