proto/stp: sans-IO spanning-tree state machine

Migrate the full non-async spanning-tree surface into proto/stp/, mirroring the
discovery/routing/fmp/mmp conversions. The classification ladder (parent-switch /
self-root / loop-drop / ancestry-update / periodic-rebroadcast / parent-lost) moves
out of the async node handlers into a pure Stp classify layer returning a
TreeDecision the shell drives, with effect ordering and per-arm invalidation
preserved verbatim. src/tree/ relocates wholesale: TreeState + ParentDeclaration data
+ coordinates into proto/stp/{state,coordinate}, the flap-dampening / hold-down
cluster into a FlapDampener in limits.rs, and the wire codec into wire.rs. The clock
is injected as u64 (wall-clock secs for the escaping declaration timestamp, monotonic
ms for the dampening timers via mmp::mono_ms); declaration crypto is field-partitioned
so sign/verify/hash run in the shell while the in-core modules carry data +
signing_bytes only. Peer maps/sets move to BTree; core/state/coordinate/limits are
core+alloc clean, with wire.rs the one std-tethered file. Behavior-neutral:
characterization tests added for the handler decision arms; convergence suite and
ci-local (36/36) green.

src/cache/coord_cache.rs

@@ -9,7 +9,7 @@ use std::collections::HashMap;
 use super::CacheStats;
 use super::entry::CacheEntry;
 use crate::NodeAddr;
-use crate::tree::TreeCoordinate;
+use crate::proto::stp::TreeCoordinate;
 
 /// Default maximum entries in coordinate cache.
 pub const DEFAULT_COORD_CACHE_SIZE: usize = 50_000;

src/cache/entry.rs

@@ -1,6 +1,6 @@
 //! Cache entry with TTL and LRU tracking.
 
-use crate::tree::TreeCoordinate;
+use crate::proto::stp::TreeCoordinate;
 
 /// A cached coordinate entry.
 #[derive(Clone, Debug)]