Resolve C# local functions by lexical scope

[Widthdom]

Summary

• resolve same-name C# local functions by their nearest complete lexical block and enclosing callable instead of treating the whole file as one family
• preserve declaration-order visibility, overload families, method groups, nested blocks, lambdas, top-level statements, and conservative incomplete-range behavior
• let parameter/local delegate values shadow local functions, keep unrelated scopes and files out of the graph, and carry the same identity through references, callers, inspect, impact, selectors, and LSP navigation
• refresh existing C# indexes through bumped extractor and reference-identity contracts

Validation

• make lint
• dotnet build CodeIndex.sln -c Release --no-restore --nologo (0 warnings, 0 errors)
• dotnet run --project tools/CodeIndex.Changelog -- check
• focused Resolve C# local functions and shadowing parameters by lexical scope #5188/Preserve innermost callable containment for C# calls, local functions, and lambdas #4840 and contract tests: 14 passed on both .NET 8 and .NET 9
• LSP suite: 128 passed on both .NET 8 and .NET 9
• DB/reference graph suite: 1,048 passed on .NET 8; 1,047 passed and 1 skipped on .NET 9
• full Release suite: 11,985 passed / 9 skipped on .NET 8 and 11,501 passed / 430 skipped on .NET 9; the sole failure on each framework exposed the Razor event-binding regression from the new local-function exclusion path, which was fixed and rerun together with the feature suite (7 passed on each framework)
• self-indexed the committed HEAD; workspace freshness, graph, reference graph, C# contracts, fold, SQL graph, and file-issue checks are current with zero extraction cap hits

Adversarial review

• Codex pass 1 found three issues: cross-file generic fallback to a local function, LSP fallback through explicit value shadowing, and returned method groups misclassified as local declarations. All were fixed with regression coverage.
• Codex pass 2 found three issues: zero-candidate LSP suppression without explicit negative evidence, lexical overload method-group leakage, and top-level local functions escaping their file. All were fixed with graph and LSP regression coverage.

Documentation and changelog

• updated the bilingual developer and testing guides for the lexical-resolution contract and regression coverage
• added changelog.d/unreleased/5188.fixed.md

Follow-ups

• None.

Fixes #5188