Resolve four confirmed unused internal helpers

[Widthdom]

Summary

StringRowsEqual, QuotedIncludeRegex, HasPrologClauseTerminator, and PreferDefinitionAtPosition have no indexed or text-validated uses in current source.

Audit IDs: R-02, R-03, R-04, and R-05. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

No exact issue covers these helpers. Closed #3671 is an adjacent earlier dead-helper cleanup; PreferDefinitionAtPosition is also adjacent to closed #4566's position/navigation work.

Expected behavior and PR scope

One small cleanup PR should decide remove-versus-reconnect for each confirmed helper based on intended behavior. Do not retain speculative dead code or silently change behavior.

Implementation guide for Codex 5.6 sol xhigh

• For each helper, inspect history/tests/call paths and record a disposition: remove, or reconnect with a focused behavior reason.
• If retained, add direct behavior coverage for diff row equality, shader quoted includes, Prolog clause terminators, or LSP position preference as applicable.
• If removed, delete associated dead regex/constants/tests/docs without broad refactors.
• Re-run locally built unused/targeted searches after the change and keep one coherent cleanup commit/PR.
• Add focused regression/contract tests as appropriate, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• All four helpers have an explicit reviewed disposition.
• No helper remains unused without justification.
• Relevant behavior tests/build pass and no unrelated subsystem refactor is included.