Fix #430/#645: compiled for await...of inside an async arrow throws InvalidCastException

[nickna]

Fixes #430 and #645 (same root cause).

Problem

In compiled mode, a for await...of over an async generator inside an async arrow function crashed:

System.InvalidCastException: Unable to cast object of type '<g>d__0' to type
'System.Collections.IEnumerable'. at <>c__AsyncArrow_0.MoveNext()

async function* g() { yield 1; yield 2; }
(async () => { for await (const x of g()) console.log(x); })();  // compiled: crash

AsyncArrowMoveNextEmitter had no EmitForOf override, so the loop fell through to the synchronous for-of path in StatementEmitterBase and cast the async-generator state machine to IEnumerable. The interpreter handled the same program correctly, and the async function form already worked.

Fix

The correct async-iterator lowering lived only inside AsyncMoveNextEmitter (with a divergent, more-limited copy in AsyncGeneratorMoveNextEmitter). Rather than add a third copy, I extracted it into StatementEmitterBase.EmitForAwaitOf so all three async state-machine emitters share one implementation:

• AsyncMoveNextEmitter / AsyncArrowMoveNextEmitter dispatch to the shared base method from their EmitForOf override when f.IsAsync.
• AsyncGeneratorMoveNextEmitter keeps a protected override (it needs its own error-propagating + strict-done variant for the async-generator-consumer case).

Loop-variable binding now routes through the DeclareLoopVariable / EmitStoreLoopVariable hooks instead of Ctx.Locals directly — the async arrow resolver reads its own private _locals map (via its RegisterLoopLocal override), so a binding registered in Ctx.Locals read back as null in the arrow body.

Validation

• Compiled output now matches the interpreter for: the $IAsyncGenerator path, custom Symbol.asyncIterator, break/cleanup (iterator.return()), captured-variable accumulation, and nested-arrow (captured-outer-state-machine) contexts.
• Emitted IL passes ILVerify for every shape.
• All 12,690 SharpTS.Tests pass (incl. the EmitterSyncTests override guard, updated for the two new overrides).
• 4 new regression tests in AsyncArrowFunctionTests (both interpreter + compiled).

Out of scope (filed separately)

• The "async generator whose body awaits a not-yet-settled (e.g. setTimeout) promise yields no compiled output" case from Compiler: for await...of over an async generator inside an async arrow emits synchronous IEnumerable iteration (InvalidCastException) #430's comment is the pre-existing blocking-GetResult / event-loop limitation already tracked by Compiled async generator hangs on a not-yet-settled (genuinely-async) await #631 (reproduces with a plain async-function driver too — not arrow-specific).
• Type checker: top-level for await...of is accepted then mis-run as sync for-of (top-level await is otherwise rejected) #672 — top-level for await is accepted by the type checker then mis-run as a sync for-of (both modes).
• Compiled: storing to a captured outer variable inside a nested async arrow emits unverifiable IL (StackUnexpected) #673 — storing to a captured outer variable inside a nested async arrow emits unverifiable IL (StackUnexpected); pre-existing, independent of for-await.

Note: the local SharpTS.Test262 compiled-baseline host aborts with the known environmental 0x80131506 CLR crash — confirmed identical on clean origin/main (stash comparison), so unrelated to this change.