fix(mobile): hand a swipe-locked drag back to the scroll past the swipe's drift limit

A scroll whose first move past the tap slop leaned sideways (a thumb arc) locked as 'swipe' for the whole contact, and SwipeHandler drops a swipe at 50 px of vertical drift, so the gesture did nothing. Past that limit the drag is now a 'y' scroll. Also brings the mobile-handlers fileoverview to the current 500 ms / 50 px limits and notes in the Mobile Guide that a selection pauses the swipe and a wide terminal pans instead.

docs/wiki/Mobile-Guide.md

@@ -37,7 +37,8 @@ a bottom sheet rather than a dropdown. On a folding phone (iPhone Duo) dialogs s
 the hinge, and opening or closing the device is treated as the device changing shape, never
 as the keyboard appearing.
 
-**Swipe left and right** on the terminal to switch sessions.
+**Swipe left and right** on the terminal to switch sessions. A visible text selection pauses
+the swipe, and a terminal wider than the screen pans sideways instead.
 
 ## The home screen