ci(test): shard suite into four time-balanced slices

[TheGreatAxios]

Summary

• Splits the CI suite into four time-balanced shards over the same path union
• Balances shards with checked-in per-file timings and adds a non-blocking one-process pollution detector

Verification

• bun run lint and bun run typecheck pass
• Full local suite passes in one process; shard slices verified locally
• CI shards and gate must be green with the slowest slice near 45s

Fixes CL-7912

[linear-code]

CL-7912

[TheGreatAxios]

Superseded by #1028 — same work rebased onto current main under the exact Linear branch name cl-7912-shard-the-ci-test-matrix-toward-30-90s-wall-clock. Closing this rename duplicate.

[TheGreatAxios]

Closing as superseded by #1028, which carries the same 4-shard design plus fixes: it keeps the required test (src) compat job this PR drops (merging this PR would delete a required protect-main context), has the stronger file-set gate test, and is rebased on current main. Merged as b509ed3.