Vitest shards by file and runs a file's tests serially in one fork, so a single spec set the floor for the whole Integration job no matter how many shards or forks it got. Cost-aware sharding (#650) balanced the shards but could not get under that floor.
Split the three specs that exceeded it: format-matrix-comprehensive (1365s) into 4 by describe, format-matrix (1130s) into 4 with Cross-format matrix striped over FORMAT_SAMPLES, and format-matrix-generated (779s) into 3 striped over TOOLS. Largest spec is now 370s. Each preamble moved verbatim into a sibling .shared.ts exposing setupMatrixApp().
Integration shards went from 20m59s/17m55s/16m33s/9m19s to 11m44s/12m31s/10m37s/11m1s.
Coverage checked, not assumed: the set of test names collected by vitest list is byte-identical across the split, 2151 before and 2151 after. Per-shard totals matched the baseline exactly at 9903 tests, 9435 passed, 468 skipped.
Vitest's BaseSequencer sorts specs by SHA1 of their path and slices an equal number of files per shard, which is blind to how long each one takes. That dropped the four costliest generated matrix specs into a single shard: 24m47s against shard 1's 3m47s.
Partition by measured cost instead, greedy longest-processing-time-first. CI wall goes 25 min to 21.3 min. The remaining ceiling is that `format-matrix-comprehensive` and `format-matrix` are each one indivisible file, and tests within a file run sequentially in a single fork.
Coverage is unchanged by construction. The partition is total and disjoint, guarded over the real spec list for shard counts 1 through 8. Per-shard totals on the PR run matched the baseline exactly: 297 files, 9903 tests, 9435 passed, 468 skipped.
