test: consolidate the byte-identity file-compare idiom into a shared helper

[nh13]

Pure test-infra hygiene. The cutover-parity tests each reimplemented the byte-identity file-compare idiom assert_eq!(read_to_string(a), read_to_string(b)) inline — 7 sites across 5 files (dedup, group, clip, retag, correct). This consolidates them into a single assert_text_files_eq(actual, expected, label) in the shared tests/integration/helpers/assertions.rs, with unit tests for the identical / divergent / missing-file cases.

Behavior-preserving: same files, same actual/expected ordering, same per-site labels — the existing cutover assertions are unchanged and the full suite stays green. The helper deliberately does not guard for non-empty content: whether an output file must carry data rows is specific to the metric being compared, so any such check stays with its caller.

Risk: command output changes: none; unsafe changes: none, and CLAUDE.md allowlist changes: none; memory bounds, queue capacity, and thread/backpressure policy changes: none.

Added assert_text_files_eq for byte-identical text-file assertions. Added tests for identical, divergent, and missing files. Replaced seven inline comparisons in cutover-parity tests while preserving file order, labels, assertion behavior, and caller-specific non-empty checks.

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Reviewing files that changed from the base of the PR and between a54a17e and 1d9b94a.

📒 Files selected for processing (6)

• tests/integration/helpers/assertions.rs
• tests/integration/test_clip_cutover_parity.rs
• tests/integration/test_correct_cutover_parity.rs
• tests/integration/test_dedup_cutover_parity.rs
• tests/integration/test_group_cutover_parity.rs
• tests/integration/test_retag_cutover_parity.rs

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Walkthrough

The changes add a shared byte-for-byte text-file assertion helper and replace direct string comparisons in cutover parity tests for metrics and histogram outputs.

Changes

Text-file parity assertions

Layer / File(s)	Summary
Text-file assertion helper tests/integration/helpers/assertions.rs	Adds assert_text_files_eq with labeled read errors and mismatch reporting. Tests cover identical files, divergent content, and missing files.
Parity test adoption tests/integration/test_*_cutover_parity.rs	Uses the shared helper for metrics TSV and family-size histogram comparisons while preserving strategy-specific failure messages.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 1d9b9

This change centralizes parity-test file comparisons without changing their behavior, so no merge-blocking production risk remains.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses the valid test: prefix, a lowercase imperative description, and accurately summarizes the shared helper change. It does not end with a period.
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.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.44%. Comparing base (a54a17e) to head (1d9b94a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #950      +/-   ##
==========================================
- Coverage   94.44%   94.44%   -0.01%     
==========================================
  Files         307      307              
  Lines      151152   151152              
==========================================
- Hits       142763   142751      -12     
- Misses       8389     8401      +12     

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[nh13]

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