Stop C# static lambdas from creating phantom declarations

[Widthdom]

Summary

• Stop C# static-lambda modifiers and parameters from becoming phantom function/property declarations.
• Cover stateful, typed/untyped, async-order, explicit-return, Unicode/escaped, multiline, nested, and argument-position forms.
• Preserve real static members, constructors, generic/local functions, explicit-interface implementations, and assigned lambda symbols.
• Bump the C# extractor contract so a normal refresh replaces stale rows.

Root cause

The multiline C# declaration candidate composer could join call arguments with a following static-lambda header, allowing declaration regexes to reinterpret argument or type tokens as member names.

Validation

• Release build: 0 warnings, 0 errors.
• Issue Do not emit phantom functions for static lambda modifiers #4453/Stop C# static lambdas from creating phantom functions and properties #4830 targeted tests: 3/3 on net8.0 and net9.0.
• SymbolExtractor suite: net8.0 1,148/1,148; net9.0 1,127 passed, 21 skipped, 0 failed.
• Full Release suite after integrating latest main: net8.0 10,506 passed, 7 skipped, 0 failed; net9.0 10,033 passed, 419 skipped, 0 failed.
• dotnet format CodeIndex.sln --verify-no-changes.
• Changelog fragment validator and diff checks.
• Codex adversarial review: 2 loops; all actionable findings fixed and regression-tested.
• Dogfood index: exact C# symbol static count is 0; index is fresh and complete with 0 extraction-cap hits.

Documentation and changelog

• Updated the English and Japanese sections of DEVELOPER_GUIDE.md and TESTING_GUIDE.md.
• Added changelog.d/unreleased/4830.fixed.md.

Fixes #4830