E2E: exercise statsai-dev exact-SHA workflow [DO NOT MERGE]

[starkdmi]

Throwaway end-to-end PR for the merged statsai-dev workflow. Do not merge. Exercises exact PR-head artifacts, concurrency cancellation, waiting, update detection, failed-build refusal, recovery, and persistent isolated data across build switches.

[starkdmi]

E2E completed successfully; closing without merge.

• A e6b324e and B 8642dae were canceled by newer pushes.
• C 63330cd: no-wait resolved exact C and exited 2 while pending; run 32266369462 built and installed exact C.
• D a816936: status reported UPDATE AVAILABLE from installed C; run 32266755372 built and installed exact D.
• E 8b96f29: intentional run 32267161266 failure was reported for exact E; no-wait exited 2, normal use exited 1, and D remained selected with no fallback.
• F fa43d4b: run 32267414701 built and installed exact F; current Linux/macOS CI is green.
• The isolated schema-17 APFS dev store kept the same identity, refresh timestamp, and 3.3 GiB logical size across C, D, and F.
• The extracted cache retained only current and previous builds.
• Selection was restored to merged main d26bcb5 before cleanup.