uitest: three checks now failing or flaking on main; the release dry-run gates on this harness

Observed across several independent main-control runs during 2026-08-06/07 review work:

- 'ctrl+v goes back to the copied view' — deterministic failure on main, every run ('the line came back but the picture did not').
- 'a toast is drawn clear of the controls' — flaky, ~2 of 5 runs (root-caused earlier: the check requires a paused picture byte-stable across a second of wall clock, which the pre-existing one-second correction walk makes a coin flip).
- 'stopped file stops its sound' — seen failing once in one reviewer's main control; absent in other baselines; new flake.

Why it matters now: cargo release's dry run gates on scripts/uitest.sh, so a flaky harness can block or falsely bless a tag. Worth a dedicated pass before the next release: fix the deterministic ctrl+v failure, make the toast check tolerant of the correction walk (or hold the walk during it), and characterize the sound check.

Filed under the owner's standing issue authorization.