refactor(pipeline): drop legacy per-hatch path (Phase 4-B, step 5)

The batched hatch pipeline has soaked through visibility + LOD; it's
correct on every file tested (Floor plan.dxf, UTM site plans, the
non-UTM regression case). Time to remove the parallel per-hatch
plumbing that's been sitting empty since step 3:

- `gpu_hatches: Vec<HatchGpu>` — never populated since
  `upload_hatches` switched to the batched builder; dropped.
- `hatch_pixel_scissors`, `hatch_skip_flags` — same story; dropped.
- `compute_hatch_scissors` — no longer has a consumer; dropped, with
  the render-time call removed from `Primitive::prepare`.
- Hatch render pass — the dead `else if` branch over `gpu_hatches`
  is gone; only the single batched `pass.draw(0..vertex_count, 0..1)`
  remains.
- `compute_hatch_lod` — no fallback path; if `gpu_hatch_batched` is
  `None` (empty hatch list) it just returns early.

`HatchGpu` itself stays — the wipeout pipeline still uses it. So do
`hatch_pipeline` / `hatch_bgl1`, kept around for that path.

`#[allow(dead_code)]` on the four storage-buffer fields in
`HatchBatchedGpu` — they exist only to keep the bind group's
referenced resources alive; nothing reads them directly.

Known gap left as a follow-up: per-hatch viewport scissor for paper-
space MSPACE viewports isn't ported to the batched path yet. Hatches
inside a paper-space viewport can render past the viewport border
until that's added. Hasn't surfaced as a visible regression in the
test files so far.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/render.rs

@@ -144,7 +144,6 @@ impl shader::Primitive for Primitive {
             pipeline.cached_epoch = cur_key;
         }
         pipeline.compute_wire_scissors(self.uniforms.view_proj, clip_size.width, clip_size.height);
-        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(queue, self.uniforms.view_proj, clip_size.width, clip_size.height);