feat: C# first-class language support

[kehoej]

Summary

Adds C# as the 6th fully supported language in Contextception, with complete extractor, resolver, analyzer integration, and benchmark validation.

What's included

Extractor (internal/extractor/csharp/)

• Regex-based extraction of using, using static, using alias, global using, and file-scoped namespace declarations
• Block/line comment handling, stops parsing at type declarations
• ExtractDefinitions for class, interface, struct, enum, record, method, property, const, and delegate signatures

Resolver (internal/resolver/csharp/)

• Auto-detects source roots from .csproj/.shproj files
• 4-strategy resolution: project namespace mapping → namespace-to-file → namespace suffix subdirectory → filename search fallback
• Same-namespace sibling resolution via ResolveAll (C#'s implicit type visibility within namespaces)
• Smart stdlib detection with override for repos containing Microsoft.*/System.* projects (Roslyn, EF Core)
• Representative file selection using stem matching, interface preference, and hash-based rotation to avoid artificial hub nodes

Analyzer integration

• IsCSharpSameNamespace flag threaded through graph construction, categorization, and signal generation
• Same-namespace sibling discovery with class prefix matching and small-namespace gating (mirrors Go/Java/Rust patterns)
• C# test project mirror detection: EFCore.Design ↔ EFCore.Design.Tests, Roslyn-style CSharp ↔ CSharpTest, Jellyfin-style renamed projects
• csharpPrefixStemMatch for test-to-subject matching when tests use shorter CamelCase names (e.g., AutomaticLineEnder → AutomaticLineEnderCommandHandler)
• C# confidence escalation in likelyModifyConfidence for class prefix match and co-change evidence
• IsTestFile and hasTestDirComponent extended for C# patterns (.Tests, .Test, *Spec.cs, TestFoo.cs)

Bug fixes found during review

• Fixed GetCSharpSiblingsInDir passing "csharp" instead of "cs" as the file extension parameter, which would have made same-namespace sibling discovery return zero results
• Fixed resolveSameNamespace using inline test detection instead of classify.IsTestFile(), missing *Spec.cs files
• Added missing C# confidence escalation paths in likelyModifyConfidence (Go, Java, and Rust had these but C# was omitted)

Benchmarks

• Tested against EF Core (5,708 files) and Jellyfin (1,971 files)
• Head-to-head comparison expanded from 6 → 7 repos, 51 → 68 files, 4 → 5 languages
• C# scoring script (scripts/score_csharp.py) with ground truth validation

Documentation

• README, CLAUDE.md, CHANGELOG, ARCHITECTURE, features.md, mcp-tutorial.md, integrations, and issue templates updated from "5 languages" → "6 languages"

Test coverage

• 496 extractor test cases (all using variants, block comments, definitions)
• 335 resolver test cases (project namespace resolution, same-namespace siblings, stdlib override)
• 44 new analyzer test cases: csharpPrefixStemMatch, isCSharpTestProjectMirror, hasTestDirComponent C# patterns, extractTestStem C# patterns, csharpProjectInfo, IsTestFile C# patterns
• 10 new classify.IsTestFile C# cases
• Full suite: 1,303 tests passing, make check clean (vet + lint + test)