perf(dataplane): fold session FSM into the shared core (v5 S2a, no fast path)

[bb7133]

Stacked on #314 (base perf/fastpath-s1); the diff here is the single S2a commit. Will rebase onto main and retarget once #314 merges.

S2a — structural foundation for the fast path, behavior-neutral, no fast path yet. Moves the session SessionFsm out of SessionLoop into the one std::Mutex that already holds the admission counters (GateInner → FsmCore { fsm, pending, open }), so a later slice can evaluate open && pending == 0 together with a speculative transition under a single lock.

• Gate gains three pure, synchronous methods that never expose a MutexGuard: state_snapshot(), apply_slow(event) -> (Result<Effects, TransitionError>, post_state) (runs on_event under the lock, returns owned effects; caller executes them outside the lock), and seal_closed() (terminal fence, sticky + idempotent).
• SessionLoop holds core: Gate instead of fsm; apply routes through apply_slow and judges the handshake deadline on the returned post-state. The BackendProbe arm hoists the state read into a local before backend_active().await so the guard never spans the await. seal_and_end seals the core at every loop-terminal path before the FSM-still-Ready teardown window.
• run_bound_session_observed passes the one session gate to the loop, so loop core + 3 gated channels (+ a later slice's engine) share it. Tests unify on one shared Gate, mirroring production.

Per the structural contracts agreed with @CodexM5: effects_executed stays on SessionLoop (counted after execute); fast_transitions + ordered action-trace deferred to later slices; S2b TODOs marked where a read-then-await decision must re-snapshot once the engine commits concurrently; the std mutex is never held across an await.

New gate tests: apply_slow/state_snapshot drive the FSM identically to a standalone SessionFsm over a handshake+command script; seal_closed sticky + idempotent.

Gates: cargo test -p dataplane all suites green; fmt --check clean; clippy --all-targets clean.

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