peer: add the per-peer FMP control state machine (unwired)

Introduce src/peer/machine.rs: a sans-IO per-peer control FSM that
consolidates the scattered handshake/rekey/timeout driver logic now
spread across node/handlers. The machine is a pure reducer —
step(event, now, index_allocator) -> [action] — that reuses the
existing FMP decision cores (establish_inbound/establish_outbound/
cross_connection_winner/poll_*) rather than reimplementing any
decision, and returns runtime-agnostic actions the driver executes.

Control-tier state only; the published send-state boundary and the
driver wiring land in following commits. The machine is terminal at
Closed — re-dial is the reconciler's, so it holds no cross-attempt
retry state.

Includes eight unit tests: inbound and outbound establish, N:1
identity crystallization, the dual-initiation tie-break,
restart-override, rekey initiator cutover, the data-plane-owned
responder cutover boundary, and liveness -> link-dead -> report-lost.
Unwired — nothing calls it yet.

src/peer/mod.rs

@@ -11,6 +11,7 @@ mod active;
 #[cfg(any(target_os = "linux", target_os = "macos"))]
 pub(crate) mod connected_udp;
 mod connection;
+pub(crate) mod machine;
 
 pub use active::{ActivePeer, ConnectivityState};
 pub use connection::{HandshakeState, PeerConnection};