* refactor(editor): separate canvas view state
- Classify shared and view-local editor state explicitly
- Let canvas surfaces render supplied view state and report their viewport
- Preserve the existing one-canvas behavior by default
* fix(canvas): preserve loading render state
* feat(editor): model split canvas panes
- Add pure recursive split-tree operations with validated sizes
- Add explicit pane registry and independent view-state cloning
- Cap visible panes and cover close and split behavior
* refactor(editor): group state ownership modules
- Move shared and view state into the editor state namespace
- Model the partition with explicit interfaces and default factories
- Derive runtime view keys from the default view object
* feat(editor): model split canvas panes
- Add pure recursive split-tree operations with validated sizes
- Add explicit pane registry and independent view-state cloning
- Cap visible panes and cover close and split behavior
* feat(editor): add split canvas views
- Render recursive pane layouts with Reka UI splitters and pane-local headers
- Route canvas input, selection, viewport state, and close actions to the active pane
- Repaint every canvas during shared document previews and cover split lifecycle in tests
* fix(editor): clean up inactive pane interactions
- Cancel drag, padding preview, and text editing state when pane focus changes
- Remove duplicate changelog entries introduced while updating master
7a02ee2c62