clang-tidy: resolve readability-const-return-type

[knoepfel]

Resolve readability-const-return-type clang-tidy warnings.

• Code quality: Removed the unnecessary top-level const from flush_gate::index() to resolve the readability-const-return-type clang-tidy warning.
• Documentation: Marked the clang-tidy check as completed with one occurrence and recorded PR #766 in the fixes report.

[coderabbitai]

📝 Walkthrough

Walkthrough

The change removes a top-level const qualifier from flush_gate::index() and updates the clang-tidy fixes report to mark the associated check as completed.

Changes

Const return type cleanup

Layer / File(s)	Summary
Accessor signature and clang-tidy record phlex/model/flush_gate.hpp, docs/dev/clang-tidy-fixes-2026-04.md	flush_gate::index() now returns an unqualified data_cell_index_ptr, and the readability-const-return-type entry records the fix and PR #766.

Estimated code review effort: 2 (Simple) | ~5 minutes

Suggested reviewers: beojan

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: resolving the clang-tidy readability-const-return-type warning.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #766      +/-   ##
==========================================
- Coverage   83.86%   83.84%   -0.03%     
==========================================
  Files         172      172              
  Lines        7203     7203              
  Branches      845      845              
==========================================
- Hits         6041     6039       -2     
  Misses        883      883              
- Partials      279      281       +2     

Flag	Coverage Δ
scripts	80.09% <ø> (ø)
unittests	85.72% <100.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
phlex/model/flush_gate.hpp	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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