Fix exact file identity in LSP navigation

[Widthdom]

Summary

LSP definition/declaration and reference navigation could select a declaration from a case-colliding indexed file and interpret its position against the requested document. Select local candidates by binary-equal indexed path before SQL limits, and reject foreign paths during position matching. Apply the same exact identity to local reference fallbacks while preserving authoritative cross-file targets and overload behavior.

Reuse the existing audit-partition equality scope, with nested/exception restoration coverage. Regression fixtures cover case collisions without a case-sensitive filesystem, literal glob/SQL path characters, directory-prefix decoys, the 50-candidate boundary, unresolved local fallbacks and cross-file navigation.

Fixes #5373

Validation

• After rebasing onto cfcb723e5, Release LSP/audit recovery tests plus Honor token-boundary mode in named-query search count summaries #5376: 182 passed on each of net8.0/net9.0.
• Full suite before the rebase, dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false: net8.0 12,617 passed / 9 skipped; net9.0 12,123 passed / 435 skipped; zero failures. Rebase resolution changed only the placement of the bilingual testing-guide additions; the issue's code/test patch is unchanged.
• Real Release stdio LSP process: the issue's fixture returns B.Leaf at line 2, characters 13–17, and its reference at line 8, characters 10–14 (zero-based).
• dotnet run --project tools/CodeIndex.Changelog -- check.
• git diff --check.
• Codex adversarial review of origin/main..HEAD, including a second round after the rebase: no blocking/actionable issues found.

Documentation and changelog

• Updated both language sections in USER_GUIDE.md, DEVELOPER_GUIDE.md and TESTING_GUIDE.md.
• Bilingual fragment: changelog.d/unreleased/5373.fixed.md.
• No schema migration or reindex is required.

Follow-up

• Use exact indexed file identity for LSP document symbols #5382