refactor(compile): single-source ILEmitter/state-machine Expr.Get static dispatch (drift fix)

[nickna]

Round 8a: closes the highest-risk item left from the July audits — the actively-diverging duplicate of the Expr.Get static-dispatch blocks.

Problem

ILEmitter.EmitCall re-inlined the module.promises.method() and Class.staticMethod() / inherited-Promise-static dispatch that TryEmitGetCalleeViaBaseClass also implements. The copies had already drifted twice:

• an earlier drift omitted the Promise-subclass arm and shipped a MyP.resolve(1) crash (the code carries a comment admitting this);
• since then, the base copy gained the Compiled: await in a method-call argument (after an earlier arg) emits invalid IL / crashes — static, instance, and optional-chain dispatch #439 await-safe argument spill the inline copy lacks, and the two boxed extra args differently (EmitBoxIfNeeded vs EnsureBoxed).

Change

• Extracted TryEmitModulePromisesMethodCall + TryEmitClassStaticDispatch into ExpressionEmitterBase.CallHelpers; both TryEmitGetCalleeViaBaseClass and ILEmitter.EmitCall now route through them.
• The boxing divergence is now an explicit, documented seam: protected virtual EnsureBoxedArg(Expr) defaults to stack-state EnsureBoxed() (state-machine behavior unchanged); ILEmitter overrides it with its richer TypeMap/literal-aware EmitBoxIfNeeded (its behavior unchanged). Both paths emit their exact prior IL — this is a dedup, not a behavior change.
• The suspension-spill branch is unreachable from ILEmitter (sync bodies contain no await/yield), so sharing it is inert there.

Tests

• New StaticDispatchBoxingTests (3 theories × interpreter/compiled): boxing-sensitive static args from a sync top-level call site (ILEmitter path), from inside an async function (state-machine path), and Promise-subclass statics from async code.
• Existing PromiseSubclassTests.ExtendsPromise_TopLevelStaticThen already pins the historical MyP.resolve(1) regression.
• Full suite: 15458 passed / 0 failed. EmitterSyncTests pass (no allowlist changes needed — the new virtual lives on the base, and only ILEmitter overrides it).