fix(runtime): restore per-instance native thread configuration (#2658)

Restore per-instance Tokio executors for standard native instances,
honoring multi_thread and multi_thread_count with a two-worker minimum
in multi-thread mode. Preserve external execution for compact hosts.

Run construction, lifecycle operations, FFI data-plane tasks, and
hot-reloaded port forwards on the selected instance executor. Keep
shutdown cleanup running when its caller is cancelled, and release
owned executors without blocking their workers on runtime teardown.

Add runtime ownership, cancellation, cross-runtime connectivity, and
hot-reload regression coverage, and document the execution model.

Validation: all 45 PR checks passed, including linting, integration
tests, and cross-platform builds.

docs/core-architecture.md

@@ -349,6 +349,14 @@ A core instance:
 `CoreProcessRuntime`, plus optional protocol and platform capabilities. The
 bundle carries capabilities, not preconstructed portable managers.
 
+The standard native factory owns one continuously driven Tokio executor per
+instance. `multi_thread` selects a multi-thread or current-thread runtime;
+`multi_thread_count` sets the worker count with the historical minimum of two.
+Construction, start/stop, and FFI data-plane tasks use that executor, independent
+of the process management runtime. Dropping its Host owner shuts down the
+executor without blocking a worker on its own termination. Compact and WASI
+Hosts retain their externally supplied execution model.
+
 Each Module owns partial-start cleanup for its internal resources.
 `CoreInstance` has one outer cancellation and recovery path for the complete
 serial startup. `Running` therefore means the Host runtime and every enabled