Replace LightExpression TryEmit parent flags plumbing with compiler-context emit stack

[Copilot]

TryEmit only received a reduced ParentFlags view plus the immediate expression, which made custom emitters context-poor and forced parent-state plumbing through many call sites. This change centralizes emit context in CompilerContext by tracking the active expression stack so emitters can inspect the full parent chain directly.

• Compiler emit context

  • Added an emit-time stack to CompilerContext for:
    • active expressions
    • corresponding parent flags
  • Exposed lightweight accessors for the current node and ancestor lookup:
    • CurrentEmitExpression
    • CurrentParentFlags
    • EmitExpressionCount
    • GetEmitExpression(int indexFromCurrent)

• Emitter flow

  • Updated EmittingVisitor.TryEmit(...) to push/pop emit context around each expression emission.
  • This makes full parent context available at every emit level without threading extra state through custom emitters.

• LightExpression intrinsic API

  • Removed the ParentFlags parent parameter from LightExpression intrinsic Expression.TryEmit(...).
  • Updated intrinsic implementations to read parent context from CompilerContext instead of relying on a caller-supplied flag snapshot.
  • This reduces TryEmit parameter surface and aligns intrinsic emitters with centralized context access.

• Regression coverage

  • Added a focused LightExpression issue test using a custom intrinsic node to verify that emit-time context exposes:
    • the current expression
    • its immediate parent
    • higher-level ancestors in order

public override bool TryEmit(ref ExpressionCompiler.CompilerContext context, ILGenerator il, int byRefIndex = -1)
{
    Asserts.AreSame(this, context.CurrentEmitExpression);
    Asserts.AreEqual(ExpressionType.Convert, context.GetEmitExpression(1).NodeType);
    Asserts.AreEqual(ExpressionType.Add, context.GetEmitExpression(2).NodeType);

    il.Emit(OpCodes.Ldc_I4, 41);
    return true;
}

[dadhi]

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