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 <[email protected]>

src/modules/view/plot_window.rs

@@ -48,6 +48,7 @@ impl CadCommand for PlotWindowCommand {
         let p1 = self.p1?.as_vec3();
         // Draw the selection rectangle.
         Some(WireModel {
+            fill_is_3d: false,
             pick_tris: Vec::new(),
             pick_tris_low: Vec::new(),
             dash_from_start: false,