* perf(ci): pack each test tier from its own timing history
Fast shards were still ordered with the latest full-suite timing artifact,
which stayed stale across compiled-CLI session reuse. Publish and download
test-timings-fast and test-timings-full separately, and keep the legacy
artifact as a fallback until a tier has its own history.
* perf(ci): drop the shared timing-history fallback
Shard lookup now reads only the artifact for the current tier. A missing
artifact leaves history unset instead of packing from the old test-timings
file.
* test(ci): look up fast history when only full timings exist
The empty-history case now requests the fast tier against a full-tier
artifact and checks that the fast lookup ran and returned nothing. A
missing tier stays a separate test that does not query.
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Co-authored-by: Cursor Agent <cursoragent@cursor.com>