Route private base fields through unified bytecode

[rogeralsing]

Summary

• admit simple base class constructors with private instance fields into production unified bytecode
• initialize/brand private instance fields before VM entry on the production constructor bridge
• update decline contract docs and tighten the private-read decline assertion around method routing

Verification

• rtk dotnet test tests/Asynkron.JsEngine.Tests -c Release --filter "FullyQualifiedNameUnifiedBytecodeProductionConstructCallTests.BaseClassConstructorWithPrivateInstanceField_UsesProductionFastPathAndBrandsInstance|FullyQualifiedNameUnifiedBytecodeProductionConstructCallTests.BaseClassConstructorWithInstanceField_UsesProductionFastPathAndInitializesBeforeBody|FullyQualifiedNameUnifiedBytecodeProductionConstructCallTests.DerivedConstructorWithInstanceField_DeclinesAndFallsBack|FullyQualifiedNameExpressionProgramCoverageMapTests.UnifiedBytecodeExpansionContract_ListsRequiredHeadingsAndCurrentEnums"
• rtk dotnet test tests/Asynkron.JsEngine.Tests -c Release --filter "FullyQualifiedNameUnifiedBytecodeProductionConstructCallTests|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* (no matches)
• rtk git diff --check

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