fix(view): unify model VPORT / paper VIEWPORT view handling

Model-space tiled VPORTs and paper-space floating VIEWPORTs each carried
their own copy of the saved-view→camera math, so a fix to one drifted
from the other (the recurring grid/twist regressions). Route both, plus
the View-table and sheet-viewport paths, through one decoder and fix the
bugs that the duplication hid.

Decoder unification:
- New camera_from_view(direction, target, center, height, twist,
  world_offset): the single saved-view→Camera decoder. camera_from_vport
  (tiled), camera_for_viewport (floating, + auto-fit), and the View-entry
  / sheet-viewport apply paths all delegate to it. Tiled vs floating now
  differ only in field source and the floating auto-fit fallback.

Twist round-trips on layout switch and file save:
- camera_from_view applies roll = -twist; the write path was dropping it,
  so model→paper→model (and reload) reset the view to raw orientation.
- New Camera::roll() recovers the camera roll; sync_camera_to_document,
  write_camera_view_entry, and vport_from_camera now store -roll as the
  view twist on the *Active VPort, the OpenCADStudio_Camera_Model View
  entry, the per-tile VPorts, and the paper sheet viewport.

Rotation-aware frustum cull:
- New view_cull_aabb(): the entity-cull rectangle is built from the
  camera's right/up basis (so a twisted/rotated view's rotated rectangle
  is enclosed) instead of world X/Y axes; returns None for tilted views
  to disable the flat-XY cull rather than wrongly hide geometry. Applied
  to the model-tile and paper-viewport cull sites.

Paper viewport scale:
- The floating-viewport auto-fit overlap test summed view_target +
  view_center raw; under a twist view_center is a DCS offset, so the sum
  landed far from the real WCS centre and wrongly tripped the auto-fit,
  rendering content at the wrong zoom. It now tests the decoded
  (twist-folded, wire-space) target. The CPU projection path derives its
  scale from that same camera, locking hit-test/snap to the GPU render.

Grid enumeration:
- ViewportInstance gains grid_on; the two grid-view collectors collapse
  into one grid_views() derived from the same active_viewports() list the
  renderer uses, so the grid overlay can no longer drift from the views
  on screen.

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

src/app/view.rs

@@ -136,13 +136,11 @@ impl OpenCADStudio {
 
             // Each view draws its own grid, clipped to its own bounds, so they
             // stay independent: model tiles in model space; the sheet plus each
-            // floating viewport (clipped to its rectangle) in paper space.
-            let grid_views = if !is_paper {
-                tab.scene.model_tile_grid_views(vw, vh)
-            } else {
-                tab.scene.paper_viewport_grid_views(vw, vh)
-            };
-            let grid: Vec<overlay::GridParams> = grid_views
+            // floating viewport (clipped to its rectangle) in paper space. One
+            // enumeration for both, shared with the renderer.
+            let grid: Vec<overlay::GridParams> = tab
+                .scene
+                .grid_views(vw, vh)
                 .into_iter()
                 .map(|(bounds, cam)| {
                     let plane = grid_plane_from_camera(cam.pitch, cam.yaw);