fix(grid): independent, clipped grid per viewport

Grid (and grid-snap) state was shared through the global show_grid flag, so
toggling it in model space also turned it on in paper space, and a floating
viewport's grid painted across the whole paper — and beyond the 3-D view when
zoomed in.

- Store grid/snap per view: a model tile in model space, the active floating
  viewport or the layout's sheet viewport in paper space, round-tripped via the
  viewport status flags.
- adopt_view_display on layout switch and viewport enter/exit so the live
  toggle follows the view you are in.
- Render one grid per view (paper_viewport_grid_views): the sheet over the
  paper, each floating viewport clipped to its screen rectangle.
- Clamp the grid clip to the widget so an overflowing viewport rectangle never
  paints grid outside the 3-D view.

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

src/ui/overlay.rs

@@ -284,11 +284,28 @@ impl canvas::Program<Message> for SelectionCanvas {
         }
 
         // ── Grid display ──────────────────────────────────────────────────
-        // One entry per pane with its grid on. Each is clipped to its own tile
-        // rectangle so lines never spill into neighbouring panes.
+        // One entry per pane with its grid on. Each is clipped to its own
+        // rectangle so lines never spill into neighbouring panes — and the clip
+        // is intersected with the widget so a zoomed-in viewport whose rectangle
+        // overflows the canvas never paints grid outside the 3-D view.
         for g in &self.grid {
             let gb = g.bounds;
-            frame.with_clip(gb, |f| draw_grid(f, g.view_proj, g.plane, gb));
+            let cx0 = gb.x.max(0.0);
+            let cy0 = gb.y.max(0.0);
+            let cx1 = (gb.x + gb.width).min(bounds.width);
+            let cy1 = (gb.y + gb.height).min(bounds.height);
+            if cx1 <= cx0 || cy1 <= cy0 {
+                continue;
+            }
+            let clip = iced::Rectangle {
+                x: cx0,
+                y: cy0,
+                width: cx1 - cx0,
+                height: cy1 - cy0,
+            };
+            // Project with the full viewport rect `gb` so the grid stays aligned;
+            // only the clip is clamped.
+            frame.with_clip(clip, |f| draw_grid(f, g.view_proj, g.plane, gb));
         }
 
         if let (Some(a), Some(b)) = (self.selection.box_anchor, self.selection.box_current) {