Add BroadcastChannel for instant same-browser tab sync — opening/closing
tabs in one window propagates immediately to sibling tabs. Extract
reconcileRemoteLayers() helper shared by BroadcastChannel and Automerge,
which cleans up cached DOM panes on remote removal and handles
active-tab-closed scenarios.
Also adds configurable rApp shortcuts (Ctrl/Alt+1-9), header swipe
gestures for rApp cycling, and body data-module-id attr for swipe context.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
