Add a RuntimeAdmin control plane for out-of-band inspection and mutation of live actors, bypassing their normal message handlers.
- src/admin.rs: add RuntimeAdmin + Admin<T> reply handle, the AdminCommand enum (ListActors, InspectActor, GetActorState, ReplaceActorState, StopActor, SuspendActor, ResumeActor), typed result types (ActorSummary/ActorStatus/ActorStateSnapshot/ListActorsResponse), and AdminError (ActorNotFound/AddressMismatch/TypeMismatch/Timeout)
- src/actor.rs: expose type-erased accessors on AnyActor — metadata() returning ActorTypeMetadata plus as_any()/as_any_mut() for downcasting — and Actor::inner()/replace_inner() so admin can snapshot or swap concrete actor state
- src/runtime.rs: wire a per-worker admin channel (admin_txs), expose Runtime::admin(), and implement the RuntimeAdmin ops that resolve an address to its owning worker, send the command, notify that worker, and return an Admin<T> handle; Admin::recv_ticking drives ticks and collects the reply
- src/worker.rs: add an admin_rx receiver and a drain_admin tick phase run before actor handlers, plus ActorPool helpers (actor_summary, get_actor_erased[_mut], suspend/resume/stop_actor_admin) that apply commands on the worker thread; fold admin_rx into the fast-idle has_work/tick_once checks
- tests/runtime_admin.rs: add 608 lines of end-to-end coverage for list/inspect/get/replace/stop/suspend/resume and the type-mismatch and not-found error paths
Signed-off-by: Zachery Aaron Shores-Chmielewski <zacheryasc@gmail.com>
