fix(canvas): stop the layer-order render loop during region drag

[SaneSanders]

Summary

• reconcileCanvasLayerOrder returns the current array when the layer order is unchanged, so setLayerOrder bails out.
• During a region drag previewDelta is recreated on every render, which gives activeLayerIds a new identity each time; with the old always-new array the layer-order effect re-rendered the canvas without end.

Test plan

• npm test (852/852)
• npm run typecheck
• Live, dragging a region on dab5479: 23 × "Maximum update depth exceeded" per gesture → 0