perf(scene): hatch LOD — solid-fill substitute + sub-pixel skip (Phase 3.3)

Hatch fills drew the full per-family pattern loop every frame regardless
of zoom. Two LOD tiers, neither of which existed:

  * **Shader-side solid substitute.** When the densest line family's
    spacing × scale projects to less than 2 px, individual lines blur
    into a solid color and the per-fragment family loop is wasted ALU
    (and produces moiré). Pass `world_per_pixel` through Uniforms; the
    hatch fragment shader now returns `h.color` directly in that case.
  * **CPU-side sub-pixel skip.** When the entire hatch AABB projects
    to less than 2 px, drop the draw call. Adds `world_aabb` to
    HatchGpu plus a per-frame `compute_hatch_lod` pass (mirrors the
    existing scissor pass) and `hatch_skip_flags`. The render pass
    skips flagged hatches.

Factored a shared `aabb_below_pixel` helper for future LOD callers.

The Uniforms layout is unchanged in size (96 B) — the trailing
`_pad: vec2<f32>` becomes `world_per_pixel: f32, _pad: f32`, so wgsl
modules that don't read the new field keep working with their old
`_pad: vec2<f32>` declarations.

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

src/scene/render.rs

@@ -147,6 +147,7 @@ impl shader::Primitive for Primitive {
         pipeline.compute_hatch_scissors(self.uniforms.view_proj, clip_size.width, clip_size.height);
         pipeline.compute_wipeout_scissors(self.uniforms.view_proj, clip_size.width, clip_size.height);
         pipeline.compute_image_scissors(self.uniforms.view_proj, clip_size.width, clip_size.height);
+        pipeline.compute_hatch_lod(self.uniforms.view_proj, clip_size.width, clip_size.height);
         if self.show_viewcube {
             pipeline.viewcube.upload(
                 queue,