Fix C# Regex alias search and unused contract annotations

[Widthdom]

Summary

• Resolve C# using-alias constructor references so references BoundedRegex --kind instantiate distinguishes BoundedRegex aliases from direct BCL Regex construction.
• Add bcl_regex_without_timeout reference reporting for direct BCL Regex construction without a timeout argument.
• Classify serialization/reflection contract annotations on public C# properties, fields, constructors, and methods as reflection_or_config_suspect in unused-symbol analysis.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --configuration Release --framework net9.0 --filter "FullyQualifiedName~ConsoleUiTests.PrintUsage_WithoutBanner_HidesAsciiArtAndEasterEggFlags"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ReferenceExtractorTests|FullyQualifiedName~DbReaderTests.GetUnusedSymbols_SerializationAndReflectionContractAttributes_AreClassifiedAsSuspect|FullyQualifiedName~DatabaseTests.InsertReferences_ExistingReferenceKinds_AreAccepted"
• dotnet build
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check origin/main..HEAD
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Fixes #3391
Fixes #3396