Plan step 35: <x-pane>, a content region with M3's margins (16px below
medium, 24px from it) drawn once however panes and layouts nest, a width
cap, and an optional pane app bar - <x-app-bar> as a direct child of the
pane with title, subtitle, actions, a back link or action, a leading
slot, and the section navigation under it.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9
