tsort: fix minimal cycle reporting and precise back-edge removal (no refactors)

[naoNao89]

This PR fixes incorrect loop reporting in tsort by reporting only the minimal cycle subpath and removing the precise back-edge that closes the cycle. No performance refactors or unrelated changes are included.

Context

• Addresses incorrect minimal cycle reporting discussed in Issue tsort: incorrect loop detection #8743
• Keeps existing DFS structure; does not introduce ID interning / CycleDetector / HashSet changes
• Excludes unrelated df test changes and GNU-compat test suite additions

What changed

• When detecting a cycle, reconstruct and report only the true cycle nodes (no non-cycle prefix from the DFS path)
• Remove the exact closing back-edge (last -> first) for the detected minimal cycle to progress deterministically
• Maintain deterministic behavior by sorting node iteration for cycle detection

Why

• Aligns error reporting structure more closely with GNU behavior by printing the minimal loop content
• Ensures subsequent traversal remains coherent when multiple cycles exist by removing the precise back-edge that closes the reported cycle

Non-goals of this PR

• No architectural/performance refactoring (no iterative DFS, no ID interning, no HashSet successor storage)
• No additional test infrastructure or GNU-compat suite changes
• No unrelated test changes (e.g., df tests)

Testing

• Ran cargo test --package uu_tsort and the top-level cargo test tests locally; both succeeded
• Existing tsort tests continue to pass

Follow-up PRs in planned split

• PR 2: Iterative DFS and ID interning for cycle detection (performance-focused)
• PR 3: Test enhancements including Linux-only GNU compatibility tests
• PR 4: Unrelated df test robustness improvements

This focused PR should be easier to review and merge, addressing the core correctness issue independently.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging #8786 will improve performances by 89.99%

_{Comparing naoNao89:pr1-tsort-minimal-cycle (f8db53b) with main (1331ff1)¹}

Summary

⚡ 4 improvements
✅ 71 untouched
🆕 2 new
⏩ 73 skipped²

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	fold_custom_width[50000]	60.8 ms	46.6 ms	+30.51%
⚡	fold_many_lines[100000]	153.1 ms	125.3 ms	+22.11%
🆕	nl_large_file[10]	N/A	128.1 ms	N/A
🆕	nl_many_lines[100000]	N/A	101.6 ms	N/A
⚡	tsort_input_parsing_heavy[50000]	12.8 s	6.7 s	+89.99%
⚡	tsort_wide_dag[100000]	135.5 ms	130.1 ms	+4.13%

Footnotes

1. No successful run was found on main (f08f4d0) during the generation of this report, so 1331ff1 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 73 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)