feat(points): point style commands + DDPTYPE dialog, viewport-aware size

Points could only render as a 1px dot — no way to change PDMODE/PDSIZE.

- PDMODE / PDSIZE commands set the header value and rebuild the glyphs.
- DDPTYPE opens a Point Style dialog: a 5x4 grid of rendered glyphs
  (shape x enclosure), a size field, and relative/absolute radios.
- Relative (<=0) PDSIZE now sizes from the current world-per-pixel so the
  marker stays a roughly constant on-screen size across zoom, rebuilding
  on zoom via the existing camera-generation cache key. Absolute (>0)
  scales with the world as before. Switching to absolute with a zero size
  seeds a positive value from the on-screen size so it stays representable.

Part of the #100 extensibility epic (point style surfaced in #106).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/view.rs

@@ -1100,6 +1100,7 @@ impl OpenCADStudio {
             AssocPrompt => (440, 210),
             Unsaved => (420, 160),
             SaveDialog => (560, 480),
+            PointStyle => (360, 470),
         };
         Some((w as f32 + EXTRA_W, h as f32 + EXTRA_H))
     }
@@ -1572,6 +1573,15 @@ impl OpenCADStudio {
                 };
                 sized(unsaved_changes_dialog_window(&tab_name), 420, 160)
             }
+            super::ModalKind::PointStyle => sized(
+                crate::ui::point_style::view_window(
+                    self.tabs[self.active_tab].scene.document.header.point_display_mode,
+                    self.point_size_relative,
+                    &self.point_size_buf,
+                ),
+                360,
+                470,
+            ),
             super::ModalKind::SaveDialog => sized(
                 save_as_dialog_window(
                     &self.save_dialog_filename,