Fix impact file projection paths

[Widthdom]

Summary

• Derive impact file-impact projection fields from the runtime FileDependencyResult model.
• Preserve source_path, target_path, reference_count, and result_kind in compact file-impact rows while rejecting the ambiguous file_impacts.path alias.
• Add regression coverage for full, compact, explicit, invalid, zero-result, cursor-paged, and byte-bounded output.

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false (0 warnings, 0 errors)
• dotnet test CodeIndex.sln -c Release --no-build -p:UseSharedCompilation=false (net8.0: 11,820 passed / 9 skipped; net9.0: 11,340 passed / 426 skipped; 0 failed)
• Focused Issue Fix impact file-impact projection so compact and --fields preserve both paths #5156 projection tests on net8.0 and net9.0 (90 passed per target)
• Impact command tests on net8.0 and net9.0 (37 passed per target)
• dotnet run --project tools/CodeIndex.Changelog -- check
• CLI smoke checks for compact endpoint identity, explicit endpoint selection, catalog output, and typed rejection of file_impacts.path
• Root index and workspace checks are healthy at the branch HEAD
• Codex adversarial review of origin/main..HEAD: no actionable findings

Documentation and changelog

• Updated the English and Japanese output contracts in DEVELOPER_GUIDE.md.
• Updated the English and Japanese test guidance in TESTING_GUIDE.md.
• Added changelog.d/unreleased/5156.fixed.md.

Follow-up candidates

• None.

Fixes #5156