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/cache/block_cache.rs

@@ -450,7 +450,7 @@ fn tessellate_sub_local(
     // top-level MTEXT outside blocks the colour split is preserved via the
     // hot path in `tessellate_entity`.
     let mut wires_out = tessellate::tessellate(
-        doc, h, sub, false, sub_color, pat_len, pat, lw_px, local_offset, anno_scale,
+        doc, h, sub, false, sub_color, pat_len, pat, lw_px, local_offset, anno_scale, None,
     );
     if wires_out.is_empty() {
         return None;