Route lexical arrow bindings through unified bytecode

[rogeralsing]

Summary

• pass captured lexical this/new.target values into production unified-bytecode VM entry for arrows
• widen simple-return arrow admission to allow LoadThis/LoadNewTarget while still declining super/environment-backed arrows
• add invocation coverage for lexical this/new.target arrows and update the expansion contract

Verification

• rtk dotnet test tests/Asynkron.JsEngine.Tests -c Release --filter "FullyQualifiedNameUnifiedBytecodeProductionInvocationTests.ArrowFunction_LexicalThis_UsesUnifiedBytecodeProductionFastPath|FullyQualifiedNameUnifiedBytecodeProductionInvocationTests.ArrowFunction_LexicalNewTarget_UsesUnifiedBytecodeProductionFastPath|FullyQualifiedNameUnifiedBytecodeProductionInvocationTests.SimpleArrowFunction_UsesUnifiedBytecodeProductionFastPath|FullyQualifiedNameUnifiedBytecodeProductionInvocationTests.ArrowFunction_CapturedThis_DeclinesUnifiedBytecodeProductionFastPath|FullyQualifiedName~ExpressionProgramCoverageMapTests.UnifiedBytecodeExpansionContract_ListsRequiredHeadingsAndCurrentEnums"
• rtk dotnet test tests/Asynkron.JsEngine.Tests -c Release --filter "FullyQualifiedNameUnifiedBytecodeProductionEligibilityTests|FullyQualifiedNameUnifiedBytecodeProductionInvocationTests|FullyQualifiedName~ExpressionProgramCoverageMapTests"
• rtk dotnet build src/Asynkron.JsEngine/Asynkron.JsEngine.csproj -c Release
• rtk rg "EvaluateExpression(|ProfileEvaluateExpression(" src/Asynkron.JsEngine/Ast/TypedAstEvaluator.ExecutionPlanRunner*
• rtk git diff --check

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