feat(render): double-single fills for the face3d pipeline

Table / PolyfaceMesh / greek-text / mleader-background fills go through
the face3d pipeline; their vertices were single f32 and quantized at UTM
scale. WireModel now carries a fill_tris_low residual (filled by the
tessellate Lines path), Face3DVertex gains position_low (location 3), and
face3d.wgsl reconstructs the eye-relative position from the pair. 3DFACE
quads built from key_vertices use a zero residual (no low source).

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

src/entities/dimension.rs

@@ -1172,6 +1172,7 @@ fn tessellate_dimension_inner(
                     plinegen: true,
                     vp_scissor: None,
                     fill_tris: vec![],
+                    fill_tris_low: Vec::new(),
                 });
             }
             if !ext2.is_empty() {
@@ -1192,6 +1193,7 @@ fn tessellate_dimension_inner(
                     plinegen: true,
                     vp_scissor: None,
                     fill_tris: vec![],
+                    fill_tris_low: Vec::new(),
                 });
             }
         } else {
@@ -1212,6 +1214,7 @@ fn tessellate_dimension_inner(
                 plinegen: true,
                 vp_scissor: None,
                 fill_tris: vec![],
+                fill_tris_low: Vec::new(),
             });
         }
     }
@@ -1233,6 +1236,7 @@ fn tessellate_dimension_inner(
         plinegen: true,
         vp_scissor: None,
         fill_tris: geom.arrow_fill,
+        fill_tris_low: Vec::new(),
     });
 
     // DIMTFILL: 0=none, 1=drawing background (mask), 2=DIMTFILLCLR.
@@ -1265,6 +1269,7 @@ fn tessellate_dimension_inner(
                     plinegen: true,
                     vp_scissor: None,
                     fill_tris: rect,
+                    fill_tris_low: Vec::new(),
                 });
             }
         }