Fix innermost C# callable containment

[Widthdom]

Summary

• select the innermost C# callable container consistently for reference and symbol extraction, including test.method, local functions, nested local functions, lambdas, expression-bodied methods, and nested types
• keep callable-first tie-breaking scoped to C#, preserve compact non-C# behavior, and handle shortened callable ranges plus primary-constructor boundaries
• add focused regressions for issue Preserve innermost callable containment for C# calls, local functions, and lambdas #4840 and both adversarial-review rounds

Validation

• dotnet build CodeIndex.sln --no-restore -c Release -m:1 -p:UseSharedCompilation=false -nodeReuse:false (0 warnings, 0 errors)
• issue/review tests: .NET 8 6 passed; .NET 9 6 passed
• Reference/Symbol extractor suite: .NET 8 2,414 passed; .NET 9 2,389 passed, 25 skipped
• full Release suite: .NET 9 10,110 passed, 419 skipped; .NET 8 10,581 passed, 7 skipped, with two unrelated load-sensitive tests failing during three concurrent repository-wide runs and then passing 2/2 in an isolated rerun
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog --no-build -c Release -- check
• git diff --check origin/main...HEAD
• two rounds of Codex adversarial review completed; all actionable findings were addressed

Documentation and changelog

• updated the English and Japanese sections of README.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md
• added changelog.d/unreleased/4840.fixed.md with bilingual notes and references to C#: callers attributes expression-bodied callers (public int Wrap() => Compute();) to the enclosing CLASS instead of the individual method/property #233 and Persist precise symbol/reference spans for LSP and editor formats #4417

Follow-up candidates

None.

Fixes #4840