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/scene/mod.rs

@@ -7109,6 +7109,7 @@ fn tessellate_entity(
             1.5,
             world_offset,
             1.0,
+            world_per_pixel,
         );
         let ab = entity_aabb(e, world_offset);
         for w in &mut wires {
@@ -7423,6 +7424,7 @@ fn tessellate_entity(
                     sub_line_weight_px,
                     world_offset,
                     anno_scale,
+                    world_per_pixel,
                 );
                 for w in &mut wires {
                     w.name = h.value().to_string();
@@ -7584,6 +7586,7 @@ fn tessellate_entity(
         line_weight_px,
         world_offset,
         anno_scale,
+        world_per_pixel,
     );
     for b in &mut bases {
         b.aci = aci;