Restore minimal view-cube turns and drag-to-orbit

Recover N3MI-DG's TOP/BOTTOM heading and drag-to-orbit contributions
from PR #101, adapted to the existing 14-target cube. Add Y-up/Z-up
regressions, navigation ownership and dialog blocking coverage.

Original heading commit: c28e780f52
Original orbit commit: f30b30be6e
Source: https://github.com/bherbruck/solvecraft/pull/101

Co-authored-by: N3MI-DG <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

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

@@ -15,8 +15,10 @@
 - **Viewport.** Click to select, Shift or Ctrl to add, drag for a window or crossing box. The
   wheel zooms at the cursor, the middle button pans and Shift+middle orbits (the mouse preset in
   Preferences switches to SolidWorks or Inventor style). The view cube and the navigation bar
-  change the view: a cube face turns to its standard view, a chamfered corner to that corner's
-  iso view, and the house goes home. Right-click for the marking menu: eight commands around the cursor, plus a
+  change the view: a cube face turns to look straight at it (TOP and BOTTOM keep the nearest
+  quarter-turn heading), a chamfered corner to that corner's
+  iso view, and the house goes home. Drag the cube to orbit around the view centre.
+  Right-click for the marking menu: eight commands around the cursor, plus a
   menu for whatever is under it.
 - **Dialogs** dock on the right with a live preview. Most features also show drag handles and a
   value box in the viewport.