test-infra: transcription-do 'voice tier' flakes under full-suite vitest-pool-workers concurrency (double-metered minutes)

[unforced]

Found during C2 review (PR #170, round 2). ~30-40% of FULL-suite bun x vitest run runs in workers/vault fail the voice-tier test with double-metered minutes (596 vs 598). Never reproduces running the file in isolation.

Bisected properly: reproduced at the identical rate at the pre-round-2 commit (4d4926f) AND traced to a TOCTOU window in transcribeOne's "is this attachment still pending" check — only reachable when the vitest pool interleaves two alarm() invocations under load. Real Cloudflare DOs serialize alarm invocations, so no production exposure is believed — but the C2 reviewer is delivering an independent judgment on that claim (will be appended here).

Impact: can occasionally flake an unrelated PR's CI. Options: (a) serialize alarm invocations in the test harness to match real DO semantics, (b) make transcribeOne's pending-check compare-and-set atomic (also hardens production against future workerd semantic changes), (c) both.

Related: the alarmRunning reentrancy guard added in #170 narrows but does not close this window under the pool's concurrency model.

[unforced]

C2 reviewer's independent judgment (delta pass, #170): TOCTOU diagnosis CONFIRMED credible from the code (transcribeOne guards on pending at entry at vault-do.ts:2225, long whisper await, done written later — two pool-interleaved alarms both pass the guard). Production exposure: NONE — real DOs serialize alarm delivery, and the write ordering (done at :2354 BEFORE metering at :2358) means a worst-case crash UNDER-meters, never double-meters. Reproduced 2/6 full-suite runs, never in isolation, never on a C2 test. Recommended de-flake: drain-fully/single-controlled-alarm pattern the embedding tests already use.

[unforced]

Escalation: this flake is now firing in CI proper (main's ci.yml vault-worker job red at 438ba40, and again on #174's PR run — the predicted spurious-red materialized within hours). It blocked nothing today only because reviews verified it pre-existing each time, but that toil compounds. Recommend the de-flake (drain-fully/single-controlled-alarm pattern) get picked up next cloud slot.