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/node/session_wire.rs

@@ -32,8 +32,8 @@
 //! | 0x2   | -      | Handshake msg2   | SessionAck (Noise XK msg2)        |
 //! | 0x3   | -      | Handshake msg3   | SessionMsg3 (Noise XK msg3)       |
 
+use crate::proto::stp::TreeCoordinate;
 use crate::protocol::{ProtocolError, decode_optional_coords};
-use crate::tree::TreeCoordinate;
 
 // ============================================================================
 // Constants