Support CustomSlotSuppliers in WorkerTuner - #156
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Introduce the public API surface for user-defined slot suppliers: CustomSlotSupplier protocol, SlotType/SlotInfo (@nonexhaustive), SlotReserveContext, SlotMarkUsedContext<Permit>, SlotReleaseContext<Permit>. Not yet wired into SlotSupplier or the worker; the following commits integrate the protocol and land the FFI bridge.
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Extend the package Kind enum with `case custom(any CustomSlotSupplier)` and add the public `custom<S>(_:)` factory so users can plug a CustomSlotSupplier into a WorkerTuner. The FFI bridge lands in the following commit; this intermediate commit does not build on its own.
BridgeCustomSlotSupplier owns a heap-allocated TemporalCoreCustomSlotSupplierCallbacks retained through user_data, and translates FFI callbacks to the user's protocol methods. Reservations are handled inside a structured task tree that races the user's reserveSlot against a cancel-watcher continuation the core resumes via its cancel_reserve callback. Permits are boxed into a PermitBox and their Unmanaged.passRetained pointer bits are used as the FFI permit ID, so the same permit value comes back in mark_used/release without a separate mapping table. BridgeWorker constructs a wrapper per custom-tuned slot supplier and retains them for its lifetime; if temporal_core_worker_new fails the wrappers are torn down explicitly via tearDownWithoutCore() so the Rust core never sees the callback pointers. The consumer task that drives each wrapper's reserve loop, and the shutdown wiring that finishes its stream, are added in the following commit.
BridgeWorkerProtocol gains `runSlotSupplierLoops()` (with a default no-op) and BridgeWorker's implementation spawns a discarding task group over its retained custom slot suppliers. BridgeWorker's initiateShutdown now calls markShutdown on each supplier so consumers exit once Rust drains their in-flight reservations. The worker's task group returns a WorkerRunTaskKind so a clean-return poller trips the existing fatalError while a clean-return slot supplier loop is expected on shutdown and drives the shutdown rerun. ReplayBridgeWorker gets an explicit no-op override — custom slot suppliers are not supported during replay — and the test forwarder protocol forwards the new requirement.
RecordingCustomSlotSupplier records every callback into a Mutex-guarded event log; the test runs a workflow end-to-end through a .custom-tuned worker and asserts reserve, markSlotUsed and releaseSlot all fire with matching permit identities.
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