Code health: MixerGainControlTests flakes in CI's Release package tests (link.identity nil) #776

MixerGainControlTests ("A linked XR18 offers gain control too (#153); no link offers none", Packages/ShowRecorderKit/Tests/MixerLinkTests/MixerGainControlTests.swift:19) failed twice in a row in CI's App (package tests (Release)) leg on PR #770, then passed on the third attempt.

• Attempt 1: https://github.com/ericdahl-dev/ShowRecorder/actions/runs/38853602319/job/116820498259
• Attempt 2: https://github.com/ericdahl-dev/ShowRecorder/actions/runs/38853602319/job/116826181985

Both times link.identity was still nil when the test gave up, after about 17.9 s. Locally it passes 3 of 3 runs in about 0.1 s.

It looks like the same family as #655: a loopback Mixer test whose link never gets an answer under CI load. That's a guess; nobody has checked the CI logs for EADDRINUSE yet.

Not fixed yet. When someone picks it up, the steps in #655 and the test stall notes (full target, strict pool, hogs) are the place to start.