Reject incompatible FTS and literal search modes

[Widthdom]

Summary

--fts --exact-substring is accepted and records both incompatible semantic flags as true, leaving clients unable to know which matching model actually ran.

Audit IDs: Q-17. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

No exact existing issue was found.

Expected behavior and PR scope

FTS and literal/exact-substring modes are one mutually exclusive option group. Keep this to parser/normalized query-context validation.

Implementation guide for Codex 5.6 sol xhigh

• Validate after aliases/defaults are normalized and before query execution.
• Return a typed usage error that explains the two modes and safe alternatives.
• Cover option ordering, other exact flags, count/output modes, JSON/human errors, and replay generation.
• Ensure query context never reports mutually exclusive modes simultaneously.
• 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

• The conflicting invocation is rejected.
• Each mode alone retains its current semantics.
• Replay/help accurately describe the applied mode.

[Widthdom]

Work started on branch fix-issue4879.