Cache architecture: identity fix, cache merge, parent-change flush

Identity cache: remove TTL-based expiry (60s TTL broke active sessions
after expiry since handle_tun_outbound checks identity_cache before
session table). Replace with LRU-only eviction bounded by configurable
identity_size (default 10K). Lookup now touches timestamp for LRU
freshness.

Cache merge: unify coord_cache and route_cache into single coordinate
cache. Both stored NodeAddr→TreeCoordinate; the layer distinction was
conceptual, not functional. Discovery-sourced entries now get the same
TTL+refresh treatment as session-sourced entries. Simplifies
find_next_hop() to single cache lookup.

Parent-change flush: clear coord_cache after recompute_coords() in both
parent-switch paths of handle_tree_announce(). Stale coordinates after
tree reconvergence cause dead-end routing that's more expensive than
re-discovery.

Tested: 493 unit tests passed, clippy clean, Docker mesh 20/20,
Docker chain 6/6.

src/node/tree.rs

@@ -205,13 +205,14 @@ impl Node {
                 return;
             }
             self.tree_state.recompute_coords();
+            self.coord_cache.clear();
 
             info!(
                 new_parent = %new_parent,
                 new_seq = new_seq,
                 new_root = %self.tree_state.root(),
                 depth = self.tree_state.my_coords().depth(),
-                "Parent switched, announcing to all peers"
+                "Parent switched, flushed coord cache, announcing to all peers"
             );
 
             self.send_tree_announce_to_all().await;
@@ -236,6 +237,7 @@ impl Node {
                 return;
             }
             self.tree_state.recompute_coords();
+            self.coord_cache.clear();
 
             let new_root = *self.tree_state.root();
             let new_depth = self.tree_state.my_coords().depth();