Stage 1 of pen bezier-handle editing — the data + command layer.
- op-editor-core: `EditorState::set_path_anchor_handle` (set / clear
an anchor's in/out handle; a `Mirrored` anchor keeps both handles
collinear) + `set_path_anchor_point_type` (switching to `Mirrored`
snaps the handles collinear). New `PathHandleSide` enum.
- op-pen-loader: `AnchorPayload` (absolute-coord anchor + resolved
handles + point-type code) on `NodePayload.path_anchors`;
`absolutize_path_anchors` now resolves the schema's anchor-relative
handle deltas into the same absolute frame as `points`.
- op-editor-ui: `SceneAnchor` + `ScenePointType` on
`SceneNode.path_anchors` so the painter + host can read the
editable handles.
op-editor-core 234 / op-pen-loader 21 / op-editor-ui 145 tests green
(+3 handle-setter units).
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