test-exec-gate-log case C flakes under load: mxbuild-log prune ties on equal mtimes #505

tests/wave2/test-exec-gate-log.sh case C ("the oldest log survived") fails intermittently under parallel load, on master too (1 of 40 runs at 10-way parallelism on c2b9bec; 1 of 15 on a PR head that does not touch this code). Serial runs pass 10/10.

Cause: exec.sh prunes mxbuild logs with ls -t … | sed -n '31,$p'. Case C creates its 35 old logs in a tight loop; under load several land in the same clock tick with equal mtimes, and ls -t breaks ties by name ascending, so 20200101-000001-1.log can sort among the newest 30 and survive the prune.

Fix options:

• fixture: give the old logs distinct mtimes (touch -t 202001010000.$(printf %02d $i)), or
• exec.sh: prune by the time-stamped file name instead of mtime (also removes the same tie in real projects, where logs written within one second can tie).

Evidence (two failing outputs) was captured during the PR #498 review.

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