fix(sequence): restore Spatial mode on touchcancel

[earino-assistant]

Summary

The sequence scrubber's changing-position restore only listens for documentMouseUp and touchEnd with zero remaining touches. An Android touchcancel (gesture takeover, notification shade, second finger still down at release) delivers neither, so _changingPosition latches true and the graph mode stays Sequence for the life of the viewer.

The consequences of the stuck mode:

• Spatial navigation arrows die — GraphService.cacheImage$ only caches spatial edges when _graphMode === GraphMode.Spatial, so in Sequence mode the spatial branch is skipped and arrows vanish.
• A full-sequence image prefetch runs on every image change — competing for mobile bandwidth and making rounds feel slow.

Fix

Merge touchCancel$ into the restore subscription in SequenceDOMRenderer.activate(), so a touchcancel is treated like a terminal zero-touch release and restores Spatial mode. TouchService.touchCancel$ is already a public getter; this is a one-line addition to the existing merge.

Test

Adds a regression test that simulates a scrubber drag (Sequence mode) followed by a touchcancel, asserting the graph mode is restored to Spatial. All 20 tests in SequenceComponent.test.ts pass.

Notes

• touchCancel$ is already wired into TouchService's other streams (touchStop$, touchesChanged$), so this is consistent with existing touch handling.
• No public API change; no new dependencies.

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