Route private named writes through unified bytecode

[rogeralsing]

Summary

• admit direct first-boundary private named property writes into production unified bytecode
• let SetNamedProperty resolve private PropertyHandle instances in the VM
• update the expansion contract and add eligibility/invocation coverage while keeping private updates/deletes declined

Verification

• rtk dotnet test tests/Asynkron.JsEngine.Tests -c Release --filter "FullyQualifiedNameUnifiedBytecodeProductionInvocationTests.PrivateNamedPropertyWrite_UsesUnifiedBytecodeProductionFastPath|FullyQualifiedNameUnifiedBytecodeProductionInvocationTests.PrivateNamedPropertyRead_UsesUnifiedBytecodeProductionFastPath|FullyQualifiedNameUnifiedBytecodeProductionEligibilityTests.Evaluate_PrivateNamedPropertyWrite_AcceptsNamedPropertyOpcode|FullyQualifiedNameUnifiedBytecodeProductionEligibilityTests.Evaluate_PrivateNamedPropertyUpdate_DeclinesWithExplicitCode|FullyQualifiedNameUnifiedBytecodeProductionEligibilityTests.Evaluate_PrivateNamedPropertyDelete_DeclinesWithExplicitCode|FullyQualifiedNameExpressionProgramCoverageMapTests.UnifiedBytecodeExpansionContract_ListsRequiredHeadingsAndCurrentEnums"
• rtk dotnet test tests/Asynkron.JsEngine.Tests -c Release --filter "FullyQualifiedNameUnifiedBytecodeProductionEligibilityTests|FullyQualifiedNameUnifiedBytecodeProductionInvocationTests|FullyQualifiedNameUnifiedBytecodeProductionConstructCallTests|FullyQualifiedNameExpressionProgramCoverageMapTests"
• 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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