Regenerate viewport dimensions when the viewport view changes

Panning, zooming, twisting or rescaling a layout viewport moves model
geometry across the sheet without touching that geometry, so nothing in
the ordinary entity-change path noticed and the dimensions drawn through
the viewport stayed where they were — detached from what they measure.

`Scene::notify_viewport_changed` announces the viewport itself as a
modified entity. That is all it takes: every trans-space reference
carries the viewport handle as the first link of its chain, so the
dependency lookup in `refresh_associative_dimensions` already finds the
associations from there, and the refresh re-places and re-measures them.

Called from each mutator that changes the paper<->model mapping: pan,
zoom, fit-to-bounds, view normalisation on entering MSPACE, the
view-direction/twist snaps, the viewcube camera path, and both
annotation-scale setters.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/scene/camera_ops.rs

@@ -567,6 +567,9 @@ impl Scene {
             viewport.view_target.x = center.x;
             viewport.view_target.y = center.y;
             viewport.view_target.z = center.z;
+            // Panning a viewport moves model geometry across the sheet; the
+            // dimensions drawn through it follow.
+            self.notify_viewport_changed(handle);
         } else {
             self.camera.borrow_mut().target = center;
         }