Remember editor pane visibility

[leevi2010-cursor]

Summary

Remember the user's editor pane choice when they use View > Hide Editor Pane or Restore Editor Pane.

• Hiding the editor enables the existing "start in preview mode" preference
• Restoring the editor disables that preference again
• Only updates the preference when the pane visibility actually changes
• Leaves preview-pane toggles and the existing last-visible-pane protection unchanged

This makes the next document window honor the most recent explicit editor-pane choice without adding another preference.

Testing

• MPPaneToggleTests: 32 tests pass
• Adds a non-headless layout test for hide, remember, restore, and clear

[schuyler]

Thanks for the PR, @leevi2010-cursor!

Issues

• Overloads a user-facing preference: hiding or restoring the editor pane silently writes editorStartInPreviewMode — the explicit "Start in preview mode" Preferences checkbox — so a user's deliberate opt-in gets cleared (and the checkbox visibly flips) as a side effect of a View-menu action, where a dedicated key or window state restoration would keep the two concepts separate.
• Untested no-op interaction: no test covers the case where the last-visible-pane guard blocks the toggle, which is the interaction most likely to silently corrupt the remembered preference.

Suggestions

• A one-line comment at the call site explaining the "write only on actual change" gate and its relationship to the last-visible-pane guard would make the coupling legible.
• The asymmetry between toggleEditorPane: (writes the preference) and togglePreviewPane: (doesn't) is unexplained, so a note on the intent would help.
• A test asserting togglePreviewPane: never touches editorStartInPreviewMode would lock in that intentional asymmetry.