SDK rewrite: delete 15 useless wrappers, replace with composables
Deleted 15 fake headless component folders (slot-prop wrappers with
no provide/inject, no DOM, no ARIA). Replaced with 9 composables:
- usePosition, useLayout, useAppearance, useTypography
- useExport, useVariables, useFillPicker, useGradientStops, usePageList
Kept 6 real headless component families with provide/inject + DOM:
- Canvas (WebGL surface lifecycle)
- ScrubInput (pointer capture + input + display)
- Toolbar (wraps Reka UI Toolbar)
- LayerTree (wraps Reka UI Tree + Atlaskit DnD)
- PropertyList (array CRUD via inject)
- GradientEditor (pointer capture for stop drag)
App components updated: use composables directly instead of wrapping
in Root components. -1047 net lines, cleaner separation.
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