fix(runtime): bind this=iterable when calling [Symbol.iterator]() (yield* sync delegation)

[proggeramlug]

Follow-up to #4777/#4783 yield* async-generator work. Fixes the this binding of the sync-iterator factory in GetIterator.

Root cause

js_get_iterator invoked the [Symbol.iterator]() factory via js_closure_call0 after clone_closure_rebind_this — which only rebinds a closure that captures this. A plain function(){ …this… } iterator factory reads this dynamically off IMPLICIT_THIS, which was left as the caller's this. So GetIterator(obj) ran the factory with the wrong this (spec: GetIterator → Call(method, obj) with this === obj).

Fix

Set IMPLICIT_THIS = iterable around the factory call (keeping the existing clone_closure_rebind_this for the captures-this effect-prototype case).

Impact

language/{expressions,statements}/async-generator: 93.8% → 94.7% (pass 833 → 841, compile-fail 0). Fixes the yield-star-sync-* thisValue ordering cluster and array-prototype destructuring iterator cases. General correctness fix for all for-of / destructuring / spread over an object whose [Symbol.iterator] factory reads this; verified no regression on for-of with a this-reading custom iterator.

Not included (return/throw delegation forwarding)

Attempted yield* .throw()/.return() forwarding to the delegated iterator (spec received.[[Type]] is throw/return). It requires the catch/return handler to re-enter the delegation loop, but Perry inlines async-generator catch bodies into the .throw() closure where there is no loop context (continue → "continue statement outside any loop"). Doing it properly is a deep state-machine rework with real regression risk to the shared yield* path (Effect / #321) — deferred.