Worker threads: closures as task callables

[nvms]

Part of #68.

A closure submitted to a pool travels as its compiled function, with every closure its body creates, serialized through the bytecode format and cached on the pool by compile name, plus its captures as a payload: use variables, $this, the scope markers, and for an arrow function the caller's variables its body names, snapshotted at submit because the VM resolves those at call time from a frame that does not exist on the other thread. Each worker loads the code once and binds a fresh instance per task. By-reference captures are refused; captures follow the same transfer rules as arguments.

Closure and anonymous class ids now come from a process-wide atomic. Worker threads compile their own bootstrap and load closures compiled on the main thread, so two compile units must never mint the same __closure_N.

Tests: tests/workers/closures.php in make workers (use captures, arrow functions, static closures, bound $this with private access, closures creating closures, repeated submits, exceptions, coercion, refusals, a closure consuming a channel). The memory soak gains repeated closure submits.