Audit unused extractor support helpers and regex declarations

[Widthdom]

Found while dogfooding the locally built cdidx on branch codex/search-audit-issues-20260619.

Duplicate check: no open issue found for unused extractor support helpers regexes CSharpReferenceExtractor Lua SQL XAML ReferenceExtractor SymbolExtractor.

Evidence from cdidx unused --json --exclude-tests --visibility private:

• src/CodeIndex/Indexer/References/Languages/CSharpReferenceExtractor.Support.cs:233 BuildCSharpUsingNamespaceScopes
• src/CodeIndex/Indexer/References/Languages/CSharpReferenceExtractor.Support.cs:327 BuildCSharpContainingTypeScopes
• src/CodeIndex/Indexer/References/Languages/CSharpReferenceExtractor.Support.cs:1773 HasActiveCSharpUsingStaticTarget
• src/CodeIndex/Indexer/References/Languages/LuaReferenceExtractor.cs:9 LuaRequireRegex
• src/CodeIndex/Indexer/References/Languages/SqlReferenceExtractor.Patterns.cs:69 QualifiedColumnReferenceRegex
• src/CodeIndex/Indexer/References/Languages/XamlReferenceExtractor.cs:18 XamlResourceReferenceRegex
• src/CodeIndex/Indexer/References/ReferenceExtractor.TypeReferences.cs:2425 ReplaceRegexMatchesWithSpaces
• src/CodeIndex/Indexer/Symbols/SymbolExtractor.Java.cs:1400 FindJavaSameLineBraceEndColumn

Problem:
Extractor support code is dense and language-specific; stale helpers make future extraction changes harder to reason about.

Acceptance criteria:

• Confirm which candidates are truly unused versus false positives.
• Remove stale declarations or add references/tests where the intended feature was accidentally disconnected.
• Keep extractor behavior unchanged unless a disconnected feature is intentionally restored.