fix: nest trashcan and zoom as separate focus trees

[mikeharv]

The basics

• I validated my changes

The details

Resolves

Fixes #9792

Proposed Changes

Make the trashcan and each zoom control their own IFocusableTree (nested under the workspace for DOM lookup), while keeping them as native Tab stops.

1. Add WorkspaceControlNavigator, a no-op navigator so arrow keys do not move focus from chrome into blocks
2. Register/unregister trash and zoom control trees with FocusManager; nest them from WorkspaceSvg.getNestedTrees()
3. Remove FOCUSABLE component registration for these controls
4. Introduce INavigator so workspace-control navigators can implement the navigator API without extending Navigator (avoids a circular import seen in the first commit)
5. Tests for Tab-restore behavior, workspace tabbability while chrome is focused, canvas/trash/zoom DOM order, and arrow no-ops from chrome

Reason for Changes

Trash and zoom live inside the workspace's focusable svgGroup_ and are also extra tab stops. This made them feel like both separate tab stops and part of the workspace. Tab skipped the workspace if a workspace control had previously been focused, and arrow keys jumped focus from the controls onto blocks.

Nesting them as separate trees fixes this.

Test Coverage

• restore previously focused block after focusing trash
• workspace root becomes tabbable again when a control is focused
• canvas -> trash -> zoom DOM order under the workspace SVG group
• arrow key from trash/zoom doesn't do anything

Additional Information

This was the first attempt at addressing the linked issue. An alternative was pursued later which removed the IFocusable implementation from the trash and zoom controls altogether. If that work is approved/merged, this branch can be closed. That work is here:

• fix: drop IFocusableNode from trash and zoom #10272