Explain candidate languages for ambiguous file extensions

[Widthdom]

Summary

Ambiguous .M lookup returns only ambiguous_m, hiding Objective-C/MATLAB candidates, evidence, and disambiguation guidance.

Audit IDs: G-35. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Follow-up to closed #4612's ambiguous-language detection work.

Expected behavior and PR scope

Return ordered candidate languages, detector evidence/rules, and explicit override guidance from the same detector used during indexing. Keep this to ambiguity diagnostics/catalog output.

Implementation guide for Codex 5.6 sol xhigh

• Expose candidate IDs/display names, confidence/evidence, content/path rules, and case behavior structurally.
• Ensure a real indexed file reports the selected language plus the same reason vocabulary.
• Cover .M/case variants, empty/binary/content-signaled files, explicit overrides, aliases, CLI and MCP.
• Avoid duplicating detection logic in the diagnostic layer.
• Add focused regression/contract tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• .M lists Objective-C and MATLAB candidates with guidance.
• Diagnostic and actual indexing decisions agree.
• Automation can choose/override using structured fields.

[Widthdom]

Work started on branch fix-issue4901.