fix(render): flag 2D overlay fills so they show in every view mode

A SOLID's filled quad and a dimension/MultiLeader text background are flat 2-D
overlays that should draw in wireframe and shaded alike, but face3d_gpu
classified fills by `!fill_tris_low.is_empty()` — and a 2-D fill at UTM scale
carries a double-single low residual for precision just like a real 3-D mesh
face. So the arrowhead fill and the "5;8" cyan text background were routed to
the 3-D buffer and only appeared in shaded mode, vanishing back to nothing on
return to wireframe.

WireModel now carries an explicit `fill_is_3d`: true only for a real surface
(PolyfaceMesh / PolygonMesh face, set in the tessellator), false for every flat
overlay fill. face3d_gpu classifies on the flag, so 2-D fills stay in the
always-visible buffer while keeping their low residual (and UTM precision).

Verified on budweiser: the `_BoxFilled` arrow SOLID and the dim '5;8' MTEXT
background both tessellate with fill_is_3d = false.

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

src/entities/dimension.rs

@@ -1352,6 +1352,7 @@ fn tessellate_dimension_inner(
             let (ext1, ext2) = split_ext_lines(&geom.ext_lines);
             if !ext1.is_empty() {
                 wires.push(WireModel {
+                    fill_is_3d: false,
                     pick_tris: Vec::new(),
                     pick_tris_low: Vec::new(),
             dash_from_start: false,
@@ -1378,6 +1379,7 @@ fn tessellate_dimension_inner(
             }
             if !ext2.is_empty() {
                 wires.push(WireModel {
+                    fill_is_3d: false,
                     pick_tris: Vec::new(),
                     pick_tris_low: Vec::new(),
             dash_from_start: false,
@@ -1404,6 +1406,7 @@ fn tessellate_dimension_inner(
             }
         } else {
             wires.push(WireModel {
+                fill_is_3d: false,
                 pick_tris: Vec::new(),
                 pick_tris_low: Vec::new(),
             dash_from_start: false,
@@ -1431,6 +1434,7 @@ fn tessellate_dimension_inner(
     }
 
     wires.push(WireModel {
+        fill_is_3d: false,
         pick_tris: Vec::new(),
         pick_tris_low: Vec::new(),
             dash_from_start: false,
@@ -1474,6 +1478,7 @@ fn tessellate_dimension_inner(
                     aci_to_rgba(&c)
                 };
                 wires.push(WireModel {
+                    fill_is_3d: false,
                     pick_tris: Vec::new(),
                     pick_tris_low: Vec::new(),
             dash_from_start: false,

src/entities/leader.rs

@@ -590,6 +590,7 @@ impl LeaderTess for Leader {
 
         if verts.len() < 2 {
             return WireModel {
+                fill_is_3d: false,
                 pick_tris: Vec::new(),
                 pick_tris_low: Vec::new(),
             dash_from_start: false,
@@ -696,6 +697,7 @@ impl LeaderTess for Leader {
         }
 
         WireModel {
+            fill_is_3d: false,
             pick_tris: Vec::new(),
             pick_tris_low: Vec::new(),
             dash_from_start: false,

src/entities/multileader.rs

@@ -1404,6 +1404,7 @@ impl MultiLeaderTess for MultiLeader {
         // WireModels so the renderer respects per-piece coloring.
         let mut wires: Vec<WireModel> = Vec::new();
         wires.push(WireModel {
+            fill_is_3d: false,
             pick_tris: Vec::new(),
             pick_tris_low: Vec::new(),
             dash_from_start: false,
@@ -1693,6 +1694,7 @@ impl MultiLeaderTess for MultiLeader {
                         xy = xy.max(y);
                     }
                     wires.push(WireModel {
+                        fill_is_3d: false,
                         pick_tris: Vec::new(),
                         pick_tris_low: Vec::new(),
                         dash_from_start: false,
@@ -1760,6 +1762,7 @@ impl MultiLeaderTess for MultiLeader {
                         wcs_corners[3],
                     ];
                     wires.push(WireModel {
+                        fill_is_3d: false,
                         pick_tris: Vec::new(),
                         pick_tris_low: Vec::new(),
             dash_from_start: false,
@@ -1795,6 +1798,7 @@ impl MultiLeaderTess for MultiLeader {
                         wcs_corners[0],
                     ];
                     wires.push(WireModel {
+                        fill_is_3d: false,
                         pick_tris: Vec::new(),
                         pick_tris_low: Vec::new(),
             dash_from_start: false,

src/entities/table.rs

@@ -753,6 +753,7 @@ pub fn tessellate_table(
     let mk =
         |color: [f32; 4], points: Vec<[f32; 3]>, fill_tris: Vec<[f32; 3]>, lw: f32| -> WireModel {
             WireModel {
+                fill_is_3d: false,
                 pick_tris: Vec::new(),
                 pick_tris_low: Vec::new(),
             dash_from_start: false,