feat(pipeline): batched-hatch GPU resources + builder (Phase 4-B, step 1)

Stand-alone module — no consumers yet. Packs an entire `&[HatchModel]`
into four storage buffers + a per-vertex (corner, instance_index)
stream:

- `InstanceBuffer`  — `HatchInstance[]` (112 B each): color, mode,
  gradient params, world_origin, local-space aabb, boundary range,
  family range, visibility flag.
- `BoundaryBuffer`  — flat `vec4<f32>[]` (NaN markers preserved as
  per-path separators, just like the per-hatch shader expects).
- `FamilyBuffer`    — flat `LineFamilyGpu[]` (matches the per-hatch
  family layout one-for-one apart from `dash_offset` indexing into a
  separate dash buffer).
- `DashBuffer`      — flat `f32[]`.

The per-vertex buffer holds `6 × instance_count` records of
`(corner: u32, instance_index: u32)`. Step 2 will add the matching
WGSL shader that uses these to project a per-instance AABB quad and
run the in-polygon test against `BoundaryBuffer[offset..offset+count]`.

iced reports 8 storage buffers per stage, 128 MB binding cap, 256 MB
buffer cap — plenty for ~10 k hatches × 112 B + ~32 MB boundaries.

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

src/scene/pipeline/mod.rs

@@ -1,4 +1,5 @@
 pub mod face3d_gpu;
+pub mod hatch_batched_gpu;
 pub mod hatch_gpu;
 pub mod image_gpu;
 pub mod mesh_gpu;