perf(producer): the audio mix runs while capture starts its browsers - #6632

What

A render's audio mix now runs while capture sets up (file server, drawElement gate probe, calibration, worker sizing) instead of before it. Capture starts as soon as both are ready.

Why

The audio stage (decode, mix, AAC encode) ran to completion before capture setup began, but nothing in setup reads its output. Audio is first needed by the disk encoder input, encode observability and the final mux. So every render with audio waited for the mix and then for the browsers, one after the other.

Related work

None.

How

• runAudioStage starts at the same point and with the same arguments, but is no longer awaited there.
• The join (await audioStage), with its perf, limiter and warning-policy handling moved unchanged, sits right after the capture plan and encoder preset resolve, before any capture path runs. So hasAudio / audioOutputPath are settled before the disk encoder input is built, exactly as before.
• If setup fails before the join, the error path waits for the in-flight mix before it reports and cleans up, so no audio process outlives the render.
• Under --strict, an audio failure still fails the render; it now surfaces after setup instead of before it.

Test plan

• Unit test added: renderOrchestrator.audioOverlap.test.ts holds the mix open and checks that worker sizing ran while it was pending, and that a setup failure is reported only after the mix settles.
• Fails without the fix: renderOrchestrator.audioOverlap.test.ts > sets up capture while the audio mixes, and a failure waits for the mix. With only the test applied, worker sizing never runs before the mix is released. With the error-path wait removed, the failure arrives while the mix is still running.
• renderOrchestrator.masterBus.test.ts stopped the render by throwing from the audio stage, which now surfaces only at the join. It now stops at capture setup's file server and still checks the master chain and video clock reach the audio stage: 1 passed.
• tsc --noEmit (producer) exit 0; new test 1 passed; renderOrchestrator.stall.test.ts 7 passed; oxfmt and oxlint clean.
• Manual testing performed: before/after on the real CLI (below).
• Documentation updated (not applicable)
• Comments follow CONTRIBUTING.md "Comments"

Before / after

Published [email protected] vs the same bundle with only this change, Apple M4 Pro (14 cores, 24 GB), hyperframes render, repo fixtures with an audio track, interleaved, 5 runs each, medians. wall - compile drops compile, which is network-bound for these fixtures (a remote video).

The CLI's stage line still reports audio with its own duration; it now overlaps setup.