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 <[email protected]>

src/modules/draw/draw/point.rs

@@ -59,3 +59,7 @@ impl CadCommand for PointCommand {
 
 // ── Autocomplete registry ─────────────────────────────────
 inventory::submit!(crate::command::CommandRegistration { names: &["PO", "POINT"] });  // PointCommand
+// Point display style system variables + dialog.
+inventory::submit!(crate::command::CommandRegistration {
+    names: &["PDMODE", "PDSIZE", "DDPTYPE"]
+});