Phase 7.3 strangler reorg — rename the web widget host crate to the
op- prefix. The crate's openpencil-shell-core dependency is repointed
to op-editor-ui (the real source crate for the widget facade / theme
/ layout scene / scene vars / render-backend / gesture types).
- crate name: openpencil-shell-web -> op-host-web
- lib name: openpencil_shell_web -> op_host_web
- every openpencil_shell_core:: path -> op_editor_ui::
- native skia.rs include_bytes! path follows the moved assets dir
- smoke harness + lib doc-comment refs updated
