Adds the canvas drag handles that author an ellipse arc — start
angle, sweep (end) angle, and the donut inner-radius.
- op-editor-ui: `ArcHandle` enum + `arc_handle_positions` (doc-space
positions of the 3 handles for an Ellipse scene node), re-exported
from `widgets`. The canvas overlay paints them as filled
primary-tinted dots for a single-selected Ellipse with the Select
tool.
- op-host-native: `ArcHandleDragState` + `arc_handle_hit` (checked
before the resize handles since the sweep grip can overlap the
right-mid resize handle). Press starts the drag with a history
snapshot; each move recomputes start/sweep/inner from the cursor
via `arc_drag_command` and re-applies `SetEllipseArc` (no-history
apply); release commits the snapshot only when the arc changed.
Dragging the start handle keeps the end fixed; a sweep collapsing
to 0 snaps to a full 360° circle.
op-editor-ui 145 / op-host-native 64 / op-host-desktop 21 tests
green (+2 arc-handle-position units).
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