feat(render): draw 3D mesh outline edges black in filled modes

Extends the solid feature-edge behaviour to mesh entities (PolyfaceMesh /
PolygonMesh): their triangulation outline edges now render black when a
filled-with-edges mode is active, and keep the entity colour in the
lines-only modes (wireframe / hidden-line).

A mesh entity emits its face fill and its outline edges as separate
WireModels sharing the entity handle: the fill carries fill_tris plus a
non-empty fill_tris_low (real 3D depth), the edge carries points. The old
is_3d_mesh_edge flag keyed on `!fill_tris.is_empty()`, which tagged the
points-less fill wire rather than the edge wire, so the edges were never
recognised. upload_wires now collects the handles of real 3D fill wires and
flags any points-wire sharing one as the mesh edge (text greek, whose fill
has an empty fill_tris_low, is excluded — matching the face3d test). The
wire draw loop then routes flagged edges through a new black wire pipeline
(fs_black) in filled-with-edges modes.

fs_black lives in wire_indexed.wgsl (the native shader) and wire.wgsl (the
wasm shader).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/shaders/wire.wgsl

@@ -206,3 +206,18 @@ fn in_dash(dist: f32, pat_len: f32, p0: vec4<f32>, p1: vec4<f32>) -> bool {
     let alpha = select(1.0, in.color.a, u.transparency_enable > 0.5);
     return vec4<f32>(in.color.rgb, alpha);
 }
+
+// Black variant: used for 3D mesh outline edges in filled render modes so the
+// mesh reads as a shaded surface framed by black edges. Keeps the dash/LOD
+// logic identical to `fs_main`; only the RGB is forced to black.
+@fragment fn fs_black(in: VertexOut) -> @location(0) vec4<f32> {
+    if in.pattern_length > 0.0 {
+        if in.min_elem >= u.world_per_pixel {
+            if !in_dash(in.distance, in.pattern_length, in.pat0, in.pat1) {
+                discard;
+            }
+        }
+    }
+    let alpha = select(1.0, in.color.a, u.transparency_enable > 0.5);
+    return vec4<f32>(0.0, 0.0, 0.0, alpha);
+}