fix(scene): hatch boundary + pattern precision for large-WCS drawings

Two precision problems were producing visibly wrong hatches on UTM-scale
(and similarly-offset) drawings even after world_offset was correct:

1. **Boundary tessellation** — `hatch_model_from_dxf`'s polyline-bulge
   arc tessellation ran the whole arc-center computation in f32. With
   vertices at WCS magnitude ~1e5 the f32 ULP is ~1 cm, which propagated
   into the sampled arc points and showed up as visibly wavy / jagged
   hatch boundaries on edges that crossed the long axis of the drawing.
   Re-run the full bulge → arc math in f64; only cast at the to_xy
   call that feeds the (also f64) boundary buffer.

2. **Pattern phase** — the per-fragment `perp / perp_step` modulo blew
   precision when `xz` was at offset-rel WCS magnitude and perp_step
   was sub-cm. Move the precision-critical bits out of f32 by keeping
   each hatch's anchor in f64 on the model side: `HatchModel` now
   carries a `world_origin: [f64; 2]` and the `boundary: [f32; 2]`
   vertices are stored as small offsets from it. The GPU vertex shader
   adds `origin` back inside the view_proj multiply for clip position
   (sub-pixel-lossy at large hatches but invisible); the `xz` varying
   that the fragment shader uses for in_polygon + pattern math stays
   in hatch-local space at full f32 precision.

   For pattern alignment across adjacent hatches with the same family,
   the GPU pipeline snaps the anchor to the family's perp/along grid
   in f64 before sending it down. Snap uses the same QCAD-PAT
   convention (`fam.dy = perp_step`, `fam.dx = along_step`) the
   shader's check_family uses — an earlier attempt used the world-frame
   `(-dx*sin + dy*cos)` formula, which produced a meaningless grid and
   no alignment.

   Trade-off: pattern phase is "global modulo per-family-grid" rather
   than tied to WCS (0,0). Same hatches still align with each other;
   the absolute phase relative to the WCS origin may differ from the
   pre-fix behaviour on small drawings.

Consumers that read `model.boundary` directly (hit_test, paper_canvas,
add_hatch round-trip) reconstruct WCS via `boundary[i] + world_origin`.
The hatch wgsl bind-group entry for binding 0 was bumped to
`VERTEX | FRAGMENT` because the vertex shader now reads `h.origin`.

The fragment-shader hatch LOD substitute + sub-pixel skip from e7675be
and the scissor plumbing from fbbac42 still work — those touched
different fields of the same struct.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/modules/home/draw/hatch.rs

@@ -119,6 +119,7 @@ impl HatchCommand {
             color: [0.75, 0.75, 0.75, 0.85],
             angle_offset: 0.0,
             scale: 1.0,
+            world_origin: [0.0; 2],
             vp_scissor: None,
         }
     }
@@ -255,6 +256,7 @@ impl GradientCommand {
             color: [0.30, 0.60, 0.95, 0.80],
             angle_offset: 0.0,
             scale: 1.0,
+            world_origin: [0.0; 2],
             vp_scissor: None,
         }
     }
@@ -404,6 +406,7 @@ impl CadCommand for BoundaryCommand {
                     color: [0.45, 0.45, 0.45, 0.60],
                     angle_offset: 0.0,
                     scale: 1.0,
+                    world_origin: [0.0; 2],
                     vp_scissor: None,
                 };
                 return CmdResult::CommitHatch(model);