Fix vacuum dry-run single-slash file URIs

[Widthdom]

Summary

• accept Unix and Windows single-slash local SQLite file URIs for vacuum --dry-run
• canonicalize query-only paths while preserving query options, ignoring URI fragments, and keeping validation and metrics on the same immutable snapshot
• add hot-WAL and URI-parity regression coverage, bilingual developer documentation, and a changelog fragment

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false (0 warnings, 0 errors)
• relevant Release URI/resolver/vacuum tests: 33 passed on .NET 8 and 33 passed on .NET 9
• enhanced vacuum regression test: passed on .NET 8 and .NET 9 after the final review fix
• full Release suite on the initial implementation: .NET 8 — 11,747 passed, 7 skipped; .NET 9 — 11,268 passed, 423 skipped; 0 failed
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check (40 fragments validated)
• local cdidx status --check and cdidx workspace status --check (fresh and healthy)
• Codex adversarial review: two rounds completed; all three actionable findings were addressed and the final follow-up diff was revalidated

Documentation and changelog

• updated both English and Japanese sections of DEVELOPER_GUIDE.md
• added changelog.d/unreleased/5123.fixed.md

Follow-up candidates

• None.

Fixes #5123