feat(viewport): tile-relative input + overlays in unified model

Route every per-frame interaction through the active model tile's screen
rectangle so a tiled VPORTS layout behaves like independent panes while
still sharing one shader widget:

- pick / draw / snap / box+lasso: cursor mapped to tile-local space; the
  hover-activates-tile rule on ViewportMove keeps the active tile under
  the cursor, so the picked camera matches what the user sees.
- pan: switched to Camera::pan_screen(dx, dy, viewport_height) so the
  world-per-pixel rate matches the active tile's height instead of the
  full canvas.
- grid: clipped via Frame::with_clip to the active tile rect; world→screen
  adds the tile origin so the lines align with the panel.
- UCS icon: origin offset by the tile rect so the gizmo sits in the
  active tile's bottom-left corner.
- render-mode picker (+ split buttons) and ViewCube hit-area: positioned
  via active_model_tile_bounds with leading Spaces, mirroring the
  floating-viewport pattern.
- ViewCube hover: driven by the CursorMoved/ViewportMove handlers into a
  new Scene::viewcube_hover cell, so the highlight survives the cube
  hit-area overlay masking events from shader::Program::update.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/grip.rs

@@ -42,8 +42,8 @@ pub fn grips_to_screen(
         .map(|g| {
             let ndc = view_proj.project_point3(g.world);
             let screen = Point::new(
-                (ndc.x + 1.0) * 0.5 * bounds.width,
-                (1.0 - ndc.y) * 0.5 * bounds.height,
+                bounds.x + (ndc.x + 1.0) * 0.5 * bounds.width,
+                bounds.y + (1.0 - ndc.y) * 0.5 * bounds.height,
             );
             (g.id, screen, g.is_midpoint, g.shape)
         })