Add Touchpad navigation: Shift to pan, Alt/Option to orbit (#1220)

An opt-in Touchpad navigation preset: Shift + pointer motion pans, Alt/Option + motion orbits, vertical scroll zooms, all without a mouse button. Existing presets are unchanged; view.navigate gains continuous motion for the control channel and scenarios.

docs/book/src/guide/shortcuts.md

@@ -42,3 +42,8 @@ Mouse and touch:
 | Double-click the middle button | go Home: the home view, framing everything shown (Preferences › Navigation › Middle double-click: Home, Fit or Off) |
 
 Help › Keyboard Shortcuts in the app lists the current bindings, including any you changed.
+
+With the **Touchpad** navigation preset, hold **Shift** while moving the pointer to pan, or
+**Alt/Option** to orbit, without pressing a mouse button. Vertical scrolling zooms. These
+viewport gestures are separate from rebindable command shortcuts; mouse-button selection
+and editing keep their existing modifiers.

docs/book/src/guide/ui-tour.md

@@ -23,13 +23,19 @@
   Esc or a click elsewhere closes the list. Clicking again on the same spot, with nothing changed
   in between, selects the next thing in that list. The
   wheel zooms at the cursor, the middle button pans and right-drag orbits freely (the mouse
-<  preset in Preferences switches to SolidWorks, Inventor, Alias or Tinkercad style). On a
+  preset in Preferences switches to SolidWorks, Inventor, Alias, Tinkercad, OpenSCAD or Touchpad style). On a
   trackpad, pinch zooms and two-finger scroll pans (Navigation › Two-finger scroll pans;
   off: scroll zooms like the wheel); mouse wheels zoom and the thumb wheel pans sideways.
   Shift+middle orbits freely
   in the default and Inventor presets. In every preset, Ctrl+left drag (Cmd on macOS) that starts
   on empty space orbits; a Ctrl+click still adds to the selection. Shift+right-drag snaps to the nearest standard or
   diagonal view; Shift does not request snapping when the preset already requires it to orbit.
+  With **Touchpad** selected in Preferences › Navigation, hold **Shift** and move the pointer to
+  pan, or hold **Alt** (**Option** on macOS) and move it to orbit—no mouse button is needed.
+  Vertical two-finger scrolling zooms in this preset, regardless of the saved two-finger pan
+  setting; switching presets restores that setting. Release the modifier to stop navigating.
+  Shift+click and Shift+left-drag still select. Combined Shift+Alt or Ctrl/Cmd modifiers do not
+  start pointer-only navigation.
   The axis view shows the world's X/Y/Z directions in SolidCraft's colors. Click a signed
   axis tip to look along that axis; click the aligned tip again to look from the opposite side.
   Drag the axis view to orbit around the view centre, with Shift for snapping. Top and bottom