Completes pen bezier-handle editing on top of the Stage-1 data model.
- canvas_viewport_paint: `flatten_path` — Path nodes with anchor
control handles render as tessellated cubic Beziers (16 steps /
segment); handle-free paths keep the straight `points` polyline.
- canvas_viewport: the Pen-tool anchor overlay now draws each
anchor's two control handles (line + dot), with a faint "ghost"
dot offset from the anchor when a handle is unset — grab it to
create the handle. `path_handle_positions` resolves real / ghost
handle positions, shared with the host hit-test.
- op-host-native: `path_anchor_hit` now distinguishes anchor body
vs handle_in / handle_out (`AnchorDragTarget`); `PathAnchorDragState`
carries the target, the anchor's fixed position, and the Shift
state. The move handler drags the anchor or a handle — a handle
drag sets the anchor's point type on first motion (Shift =
independent/broken, else mirrored/smooth) so `set_path_anchor_handle`
mirrors the opposite handle. Release commits history only on
actual motion.
op-editor-ui 148 / op-host-native 64 / op-host-desktop 21 /
op-pen-loader 21 tests green (+3 flatten-path units).
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