[Bug]: [P2] Screenshot check 'features' is intermittently nondeterministic on 27c-elevator-schedule-unsaved.png

The pinned-container determinism guard (two independent renders per shard) fails intermittently with:

screenshot check 'features' is nondeterministic: pixel bytes differ for docs/screenshots/27c-elevator-schedule-unsaved.png

Observed twice on PR #759 (runs on 2026-08-05, e.g. run 31015784833) with clean passes on intermediate pushes of the same branch, so it is probabilistic, not drift and not diff-related (#759 touches no render path). Always the SAME screenshot, which points at a wall-clock or RNG read that leaks into the 27c scene's render only when timing lands a certain way: the scene opens the elevator schedule dialog with unsaved changes, so suspects are a time-driven decoration ticking across a frame boundary between the two legs, an animation whose phase is captured mid-transition, or the unsaved-changes state depending on something time-seeded.

Until fixed this flake blocks merges at random (the determinism guard is the hard gate, correctly). Reruns pass; that is a workaround, not a fix.

Fix shape: reproduce locally in the pinned container by rendering the 27c scene twice in a loop and diffing; then pin whatever leaks (freeze the clock for the scene, or advance the sim to a stable frame before capture) the way the other scenes already do.

Start in the pinned-container rendering workflow and render docs/screenshots/27c-elevator-schedule-unsaved.png twice in a loop, diffing the pixel output. Investigate the 27c scene's clock, animation phase, and unsaved-changes state; done means repeated independent renders are byte-identical and the features determinism guard passes without relying on reruns.