Ellipse nodes with authored arc geometry (`start_angle` /
`sweep_angle` / `inner_radius`) were always painted as a full oval —
the arc fields never reached the painter.
- NodePayload + SceneNode gain `arc_start_angle` / `arc_sweep_angle`
/ `arc_inner_radius`; threaded through `ellipse_to_payload` and
`node_payload_to_scene`.
- canvas_viewport_paint: new `arc_polygon` tessellator (pie wedge =
centre + outer arc; donut sector = outer arc + reversed inner arc)
+ `paint_ellipse` — full oval when no arc is authored, otherwise a
filled/stroked polygon. A 360° sweep with no donut hole still
short-circuits to the plain oval path.
Prep for the arc canvas drag-handle UI. op-editor-ui 143 /
op-pen-loader 21 / op-host-desktop 64 tests green.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
bfbe3b161d