Relocate the widget facade (widgets/, including the Widget trait,
render primitives, the editor-UI compositions, the CanvasViewport
center canvas, the lucide icon drawer, and editor_state_ext), the
theme tokens, the layout-resolved render scene (layout_scene +
layout_scene_hit), and the design-variable aggregation (scene_vars)
out of openpencil-shell-core into a dedicated op-editor-ui crate.
The canvas widgets (canvas_viewport*) stay inside op-editor-ui rather
than splitting into a separate op-canvas crate: they depend on the
widgets/ siblings editor_state_ext + icons and on the Widget trait, so
a clean mechanical split is not possible — per the task's explicit
allowance not to force a fragile split.
op-editor-ui's lib.rs mirrors the old shell-core crate-root re-exports
(render_backend facade types + jian gesture types + the i18n alias) so
every intra-module `crate::Color` / `crate::theme` / `crate::widgets`
path resolves unchanged — a pure relocation with no path rewrites
inside the moved modules. openpencil-shell-core becomes a thin
re-export shim (`pub use op_editor_ui::{widgets, theme, ...}`) so the
hosts keep resolving `openpencil_shell_core::*` until the Task 7.3
host rename dissolves the crate. The widgets_static integration test
moves to op-editor-ui/tests with its imports rewritten. The widget
boundary script's reverse-check path is updated to the new crate.
No behaviour change; all tests move with their code.
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