The full suite was the last job to finish in 33 of 43 CI runs since it
moved from a 96-core to one 32-core runner (p50 666 s). That runner is
~90% packed, so only more machines help, and one bigger box is poor
value: on 96 cores the same files took 25-50% longer each and the suite
ran ~1.8x faster for 3x the price. Two 32-core slices scale linearly
(simulated from per-file CI timings: 262 s / 252 s vs 514 s on one box).
macOS (free 3-core runner, ~7.5 min) would become the slowest required
job once Linux splits, so it runs as two slices on the same free runner.
--slice now partitions from the checkout alone (sorted round-robin):
each slice job restores its own duration cache, and a main push saving
a newer cache between the two restores made timed LPT compute different
partitions per job, silently dropping or double-running files.
Round-robin lands within 5 s of timed LPT at this suite size. Each slice
saves its own cache key on main; the runner keeps the restored entries
for files outside its slice, so the cache stays complete.
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