perf: dedicated Face3D shader — N draw calls → 2

3DFACE entities previously created one WireGpu buffer per entity,
resulting in N separate draw calls per frame. For files with large
triangle meshes this was the dominant GPU CPU overhead.

New pipeline:
- face3d.wgsl: simple flat-color vertex + fragment shader (MVP only,
  no quad expansion, no pattern distance computation)
- Face3DGpu: batches ALL Face3D entities into a single TriangleList
  buffer (6 vertices per face, per-vertex color) — 1 draw call
- WireGpu::from_batch: merges N Face3D edge wire models into one
  quad-expanded buffer for edge rendering — 1 draw call
- split_face3d_wires: separates face3d from other wires at Primitive
  build time via document handle lookup (O(N), once per epoch)

Hit-testing via WireModel (CPU) is unaffected — face3d wires remain
in entity_wires_arc() and hit_test_wires() as before.

Before: N 3DFACE → N draw calls, N × 96 B/vertex WireVertex
After:  N 3DFACE → 2 draw calls (fill + edges), 28 B/vertex fill

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/shaders/face3d.wgsl

@@ -0,0 +1,37 @@
+// Face3D shader — flat-shaded triangle fill for DXF 3DFACE entities.
+//
+// Vertex layout (28 bytes):
+//   position  [f32; 3]   offset  0   12 B
+//   color     [f32; 4]   offset 12   16 B
+
+struct Uniforms {
+    view_proj: mat4x4<f32>,
+    viewport:  vec2<f32>,
+    _pad:      vec2<f32>,
+};
+
+@group(0) @binding(0)
+var<uniform> u: Uniforms;
+
+struct VertexIn {
+    @location(0) position: vec3<f32>,
+    @location(1) color:    vec4<f32>,
+};
+
+struct VertexOut {
+    @builtin(position) clip_pos: vec4<f32>,
+    @location(0)       color:    vec4<f32>,
+};
+
+@vertex
+fn vs_main(v: VertexIn) -> VertexOut {
+    var out: VertexOut;
+    out.clip_pos = u.view_proj * vec4<f32>(v.position, 1.0);
+    out.color    = v.color;
+    return out;
+}
+
+@fragment
+fn fs_main(in: VertexOut) -> @location(0) vec4<f32> {
+    return in.color;
+}