fix(text): canonicalise TTF filename font refs in MTEXT inline overrides

resolve_font previously returned &str, passing raw stems like "arial" verbatim to Face::resolve. System font lookup requires the canonical family name ("Arial"), so inline \f/\F overrides from block-nested MTEXT were silently unresolvable and fell through to no geometry.

Change resolve_font to return Cow<'a, str>:
- no override -> Borrowed(base), zero allocation
- LFF built-in -> Borrowed(font), zero allocation
- system font resolved -> Owned(canonical), one allocation
- unresolvable -> Borrowed(base), falls back to style font

All 4 call sites updated to &font_name (&Cow<str> derefs to &str).

Adds two unit tests covering the fallback and TTF-stem-to-family-name paths. Adds integration test verifying block-nested MTEXT produces geometry and correctly separates outline/fill wires.

tests/text_font_rendering.rs

@@ -0,0 +1,64 @@
+use acadrust::entities::{Insert, MText};
+use acadrust::tables::{BlockRecord, TextStyle};
+use acadrust::types::Vector3;
+use acadrust::{CadDocument, EntityType, Handle};
+use OpenCADStudio::scene::cache::block_cache::{expand_insert, BlockCache};
+
+fn drawable_point_count(wires: &[OpenCADStudio::scene::WireModel]) -> usize {
+    wires
+        .iter()
+        .map(|w| w.points.iter().filter(|p| p[0].is_finite()).count() + w.fill_tris.len())
+        .sum()
+}
+
+#[test]
+fn block_nested_mtext_uses_its_style_font() {
+    let mut doc = CadDocument::new();
+
+    let mut style = TextStyle::new("SHOP");
+    style.font_file = "arial.ttf".to_string();
+    doc.text_styles.add(style).unwrap();
+
+    let br_h = Handle::new(doc.next_handle());
+    let mut br = BlockRecord::new("LABEL_BLOCK");
+    br.handle = br_h;
+    doc.block_records.add(br).unwrap();
+
+    let mut mtext = MText::with_value("FERRAGAMO", Vector3::new(0.0, 0.0, 0.0));
+    mtext.style = "SHOP".to_string();
+    mtext.height = 20.0;
+    mtext.rectangle_width = 0.0;
+    let mut sub = EntityType::MText(mtext);
+    sub.common_mut().owner_handle = br_h;
+    doc.add_entity(sub).unwrap();
+
+    let ins = Insert::new("LABEL_BLOCK", Vector3::new(100.0, 50.0, 0.0));
+    doc.add_entity(EntityType::Insert(ins.clone())).unwrap();
+    let cache = BlockCache::build(&doc, 1.0, [0.0, 0.0, 0.0, 1.0]);
+    let wires = expand_insert(
+        &cache,
+        &ins,
+        Handle::new(999),
+        [1.0, 1.0, 1.0, 1.0],
+        0.0,
+        [0.0; 8],
+        1.0,
+        false,
+        1.0,
+        None,
+        None,
+        false,
+        [0.0, 0.0, 0.0, 1.0],
+    )
+    .expect("block defn is cached");
+
+    assert!(
+        drawable_point_count(&wires) > 0,
+        "block-nested MTEXT should render through its text style font"
+    );
+
+    assert!(
+        wires.iter().all(|w| w.points.is_empty() || w.fill_tris.is_empty()),
+        "outline and fill wires should be separate for correct GPU classification"
+    );
+}