FlatExpression: track nested-lambda closure parameter usage during flattening

[Copilot]

FlatExpression already indexed structural nodes (lambdas/blocks/gotos/labels/try), but did not record per-lambda captured parameter usage needed for closure-aware nested-lambda handling. This PR adds closure-usage metadata at construction time so nested lambda capture information is available without a separate traversal.

• New closure-usage metadata in ExprTree

  • Added LambdaClosureParameterUsage (LambdaNodeIndex, ParameterId).
  • Added ExprTree.LambdaClosureParameterUsages to store captured-parameter mappings per lambda node.
  • Uses FE-style short indexes and struct layout packing for compact storage.

• Capture analysis wired into FromExpression lambda construction

  • During builder processing of each ExpressionType.Lambda, collect parameters referenced by the lambda body that are:
    • not declared as that lambda’s parameters, and
    • not declared as local block/catch variables in that lambda scope.
  • Nested lambdas are intentionally excluded from the parent’s capture walk (each lambda is analyzed independently when visited).
  • Collector uses reference-based hash sets for O(1) membership checks while preserving reference-identity semantics of ParameterExpression.

• Focused unit coverage

  • Added tests for:
    • mapping outer parameter capture to nested lambda node,
    • excluding nested-lambda locals from closure usage mapping.

[StructLayout(LayoutKind.Sequential, Pack = 2)]
public readonly struct LambdaClosureParameterUsage
{
    public readonly short LambdaNodeIndex;
    public readonly short ParameterId;
}

[dadhi]

@copilot we are using short indexes in FE no need for int, apply stuct layout too.

[Copilot]

@copilot we are using short indexes in FE no need for int, apply stuct layout too.

Updated in commit b678694: switched LambdaClosureParameterUsage indices to short, applied struct layout (Sequential, Pack=2), and adjusted short conversions/usages accordingly.

[dadhi]

There are warnings and errors to fix in ci:

Warning: /home/runner/work/FastExpressionCompiler/FastExpressionCompiler/test/FastExpressionCompiler.IssueTests/Issue500_IndexOutOfRangeException_with_value_objects_implicit_conversions.cs(27,85): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. [/home/runner/work/FastExpressionCompiler/FastExpressionCompiler/test/FastExpressionCompiler.IssueTests/FastExpressionCompiler.IssueTests.csproj::TargetFramework=net8.0]
Warning: /home/runner/work/FastExpressionCompiler/FastExpressionCompiler/test/FastExpressionCompiler.IssueTests/Issue500_IndexOutOfRangeException_with_value_objects_implicit_conversions.cs(67,80): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. [/home/runner/work/FastExpressionCompiler/FastExpressionCompiler/test/FastExpressionCompiler.IssueTests/FastExpressionCompiler.IssueTests.csproj::TargetFramework=net8.0]
Error: /home/runner/work/FastExpressionCompiler/FastExpressionCompiler/src/FastExpressionCompiler.LightExpression/FlatExpression.cs(58,12): error CS1591: Missing XML comment for publicly visible type or member 'LambdaClosureParameterUsage.LambdaClosureParameterUsage(short, short)' [/home/runner/work/FastExpressionCompiler/FastExpressionCompiler/src/FastExpressionCompiler.LightExpression/FastExpressionCompiler.LightExpression.csproj::TargetFramework=net472]

[dadhi]

@codex please setup env and proceed work on it

[chatgpt-codex-connector]

To use Codex here, create an environment for this repo.

[dadhi]

@codex Env is created