fix(text): render TTF glyphs as solid fills, fix font fallback, correct Z-order

Text elements previously rendered as hollow wireframe outlines and sometimes
fell back to the wrong system font due to case-sensitive exact matching. Also,
the viewport background grid drew on top of solid entities.

Case-insensitive and partial-name fallback matching (e.g., mapping "arialn"
to "Arial Narrow") is now added to `sysfont::canonical_family_name`, ensuring
text resolves to the correct installed system font.

TrueType font contours are now triangulated into solid fills (`fill_tris`)
using `lyon_tessellation` in `ttf_glyph.rs`. To prevent the GPU from misclassifying
this 2D text as a 3D mesh (which broke flat shading), text tessellation splits
the output into separate outline and fill `WireModel`s.

The background grid is extracted from `SelectionCanvas` into `GridCanvas` and
moved beneath the 3D viewport to ensure proper layering. Finally, `DepthBiasState`
is enabled on all face/fill pipelines to apply a polygon offset, perfectly
resolving Z-fighting between coplanar wireframe lines and solid faces.

src/entities/text.rs

@@ -137,7 +137,7 @@ fn to_truck(t: &Text, document: &acadrust::CadDocument) -> TruckEntity {
         anchor_f64[1] - (anchor_local_x as f64 * sin_r + anchor_local_y as f64 * cos_r),
     ];
     // Strokes are in glyph-local space (origin = [0,0]).
-    let strokes = lff::tessellate_text_ex(
+    let (strokes, fill_tris) = lff::tessellate_text_ex(
         [0.0, 0.0],
         t.height as f32,
         rotation,
@@ -151,6 +151,7 @@ fn to_truck(t: &Text, document: &acadrust::CadDocument) -> TruckEntity {
             strokes,
             origin,
             color: None,
+            fill_tris,
         }]),
         snap_pts: vec![(snap_pt, SnapHint::Insertion)],
         tangent_geoms: vec![],