ThreadPool/ThreadChannel: no test coverage for transferring $this-bound closures (incl. method-as-closure)

[EdmondDantes]

Clients reported a bug when transferring a closure bound to $this. We have an extensive $this test matrix (19 tests, tests/thread/053–071) — but it only covers spawn_thread. ThreadPool::submit and ThreadChannel::send are separate transfer paths and currently have zero $this coverage.

Coverage gap

• tests/thread_pool/ — closure tests cover only use() vars (024), class type-hints (029/031-union), complex op_array features (031). None bind $this.
• tests/thread_channel/037 — closure transfer, but not $this-bound.

Current behavior (empirical)

The ThreadPool path works today for a basic $this closure — deep-copy of $this, parent unchanged, requires a bootloader to declare the class on the worker (same as spawn_thread):

worker: n=42 tag=hi class=Worker
result=999
parent n=42

But because nothing is tested, we cannot rule out divergence on specific sub-cases.

Tasks

1. Port the full $this matrix to ThreadPool::submit, mirroring tests/thread/053–071:
   • base property access + mutation isolation
   • private / protected properties (the original client repro, 055)
   • $this self-cycle ($this->self === $this)
   • readonly property
   • enum instance as $this + enum-typed property
   • WeakReference property (target reachable / unreachable)
   • typed/strict properties
   • missing class without bootloader → clear error, no SEGV
2. Method-as-closure cases (class refers to its own method as a closure):
   • first-class callable from instance method: $this->method(...) / $obj->method(...)
   • Closure::fromCallable([$this, 'method'])
   • Closure::bind to an object before submit
   • arrow fn fn() => $this->x
   • nested closure inheriting $this
   • first-class callable from a static method (no $this)
3. Add $this-bound + method-as-closure coverage to ThreadChannel::send.
4. Reproduce the client's exact case to confirm whether any sub-case (non-transferable property, cycle, private props, method-as-closure) diverges from spawn_thread.