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.

test/session-swipe-gesture.test.ts

@@ -147,6 +147,18 @@ describe('terminal touch handler hands sideways drags to the swipe', () => {
     expect(TERMINAL_SOURCE).toContain("panAxis = travelX > travelY ? (canPanHorizontally() ? 'x' : 'swipe') : 'y';");
     expect(TERMINAL_SOURCE).toContain('this._terminalGestureAxis = panAxis;');
     // A 'swipe' is neither a scroll nor (didScroll is set) a tap.
-    expect(TERMINAL_SOURCE).toContain("if (panAxis === 'swipe') return;");
+    expect(TERMINAL_SOURCE).toContain("if (panAxis === 'swipe') {");
+  });
+
+  it("hands a 'swipe' drag back to the scroll once it passes the swipe's own drift limit", () => {
+    // A thumb arc whose first move leans sideways (+9 px across, +5 px down) locks
+    // as 'swipe'; SwipeHandler drops it past maxVerticalDrift, so without the
+    // handback the 300 px of downward travel that follows would do nothing.
+    const start = TERMINAL_SOURCE.indexOf("if (panAxis === 'swipe') {");
+    const block = TERMINAL_SOURCE.slice(start, TERMINAL_SOURCE.indexOf("if (panAxis === 'x') {", start));
+    expect(block).toContain('SwipeHandler.maxVerticalDrift');
+    expect(block).toContain('if (Math.abs(touchY - touchStartY) <= drift) return;');
+    expect(block).toContain("panAxis = 'y';");
+    expect(block).toContain("this._terminalGestureAxis = 'y';");
   });
 });