CI speed: compile once, a sized merge queue, and teardown of cancelled jobs #5318

Scope: CI throughput rows from the look-ahead lanes: one workspace test build shared by shards, merge-queue group size and retries, verdict-preserving test speedups, job moves and opt-levels, and containers that outlive cancelled runs.

Why it matters: Every PR waits on CI; faster CI with the same verdicts is merges per hour.

Done when: The workspace test job builds once and fans out shards from one archive, cancelled runs leave no containers, and median PR CI time drops with no change in verdicts on the same commits.

Epic: #4540. Folded under triage rule 7b (2026-10-14): each row below was its own issue, closed as not planned and kept here as a sub-ticket. Reopen an issue when its work starts or its PR opens.

Sub-tickets

• SPEED-FOLD: verdict-preserving CI/test perf on car/0.70.0 #4505 SPEED-FOLD: verdict-preserving CI/test perf on car/0.70.0 (milestone backlog)
  • accept: Done when:
• FLOW-003 QM-02: merge queue group size 1 / build 8 + merge_group retries=0 + lint (after rc.1) #4515 FLOW-003 QM-02: merge queue group size 1 / build 8 + merge_group retries=0 + lint (after rc.1) (milestone 0.71.0)
  • accept: the outcome in the title, with a receipt
• FLOW-003 QM-09: compile-once — one --workspace test build, nextest archive, K shards fanned across all runners (FLOW-003 v2.1 §row QM-09) #4527 FLOW-003 QM-09: compile-once — one --workspace test build, nextest archive, K shards fanned across all runners (FLOW-003 v2.1 §row QM-09) (milestone 0.72.0); branch la-72/4527-compile-once
  • accept: the outcome in the title, with a receipt
• CI rule-3 fixes: x86-slow-2 cbtop move, sha2 opt-level 3, verify-ml python3 skip #4581 CI rule-3 fixes: x86-slow-2 cbtop move, sha2 opt-level 3, verify-ml python3 skip (milestone backlog)
  • accept: Done when: fix(build): read in-tree contracts/, not the gone provable-contracts sibling (#4369, #4380) #4554 lands, folded with --ticket PMAT-4380,<this>, each fix with its mutation/planted-RED proof.
• ci: sov.* fat-job containers outlive cancelled runs (no teardown on cancel) #4948 ci: sov.* fat-job containers outlive cancelled runs (no teardown on cancel) (milestone none)
  • accept: Done when: