fix(viewport): frame paper viewports on the robust cluster centre

Floating viewports on a survey-coordinate drawing rendered blank: the
model camera was correct, but content viewports whose saved view was
rejected auto-fitted onto empty space.

Two causes, both in model-space extents handling:

- compute_model_space_extents tested vertex finiteness in f64 but stored
  f32. A long RAY/XLINE endpoint is a huge-but-finite f64 that overflows
  to inf in f32, poisoning the extents to (inf, inf). Also, the wire/mesh
  AABB paths checked only x while folding y in, so a vertical ray's
  infinite y leaked through. Now every component is validated after the
  f32 cast.

- camera_for_viewport framed its overlap test and auto-fit on the raw
  extents centre. When a drawing has a second, far cluster (a small-
  coordinate legend beside a UTM survey), that centre sits in the empty
  gap, so a valid saved view fails the overlap test and the auto-fit aims
  at blank space. cluster_extent_from_centers already computed a robust
  centre (median of entity centroids) but discarded it; surface it as
  Scene.local_center and frame on that instead.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/update.rs

@@ -634,6 +634,7 @@ impl OpenCADStudio {
 
                 // Caches were built on the background thread inside open_path().
                 self.tabs[i].scene.local_extent_max = caches.local_extent_max;
+                self.tabs[i].scene.local_center = caches.local_center;
                 self.tabs[i].scene.hatches = caches.hatches;
                 self.tabs[i].scene.images = caches.images;
                 self.tabs[i].scene.meshes = caches.meshes;