fix(block-cache): address PR211 review follow-ups

- expand_defn: dedup the <1px baseline LOD branch the same way the <5px
  greek branch does, so colour-split MTEXT no longer stacks N overlapping
  baselines (one per \C segment) over the same OBB.
- sysfont: memoise canonical_family_name process-wide. resolve_font calls
  it once per word on the MTEXT measure hot path and Face::resolve re-runs
  it right after; the fontdb query + linear family scans were uncached.
- dimension/multileader/table: reword the 6 fill_tris_low FIXMEs. They
  cited a stale line and the wrong consumer — these fills render on the
  top-level path (panic-safe .get().unwrap_or), not the block cache, so
  they cannot trip emit_wire's debug_assert. State the real status:
  latent f32-precision debt, not a crash. Also fix the mis-indented
  text-fill WireModel in dimension.rs.
- tests: add a host-independent colour-split test (two \C segments in a
  block keep ≥2 distinct wire colours — guards the per-wire colour fix)
  and a TTF-gated UTM fill test (paired, non-zero fill_tris_low). Note the
  arial-style test's LFF-fallback limitation.

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

tests/text_font_rendering.rs

@@ -3,20 +3,28 @@ use acadrust::tables::{BlockRecord, TextStyle};
 use acadrust::types::Vector3;
 use acadrust::{CadDocument, EntityType, Handle};
 use OpenCADStudio::scene::cache::block_cache::{expand_insert, BlockCache};
+use OpenCADStudio::scene::WireModel;
 
-fn drawable_point_count(wires: &[OpenCADStudio::scene::WireModel]) -> usize {
+fn drawable_point_count(wires: &[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() {
+/// Build a one-block document holding a single MTEXT and expand the INSERT
+/// through the block cache, returning the finalized wires. `mtext_at` is the
+/// MTEXT's position inside the block; `insert_at` is where the block is placed.
+fn expand_block_mtext(
+    value: &str,
+    font_file: &str,
+    mtext_at: Vector3,
+    insert_at: Vector3,
+) -> Vec<WireModel> {
     let mut doc = CadDocument::new();
 
     let mut style = TextStyle::new("SHOP");
-    style.font_file = "arial.ttf".to_string();
+    style.font_file = font_file.to_string();
     doc.text_styles.add(style).unwrap();
 
     let br_h = Handle::new(doc.next_handle());
@@ -24,7 +32,7 @@ fn block_nested_mtext_uses_its_style_font() {
     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));
+    let mut mtext = MText::with_value(value, mtext_at);
     mtext.style = "SHOP".to_string();
     mtext.height = 20.0;
     mtext.rectangle_width = 0.0;
@@ -32,10 +40,10 @@ fn block_nested_mtext_uses_its_style_font() {
     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));
+    let ins = Insert::new("LABEL_BLOCK", insert_at);
     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(
+    expand_insert(
         &cache,
         &ins,
         Handle::new(999),
@@ -50,7 +58,22 @@ fn block_nested_mtext_uses_its_style_font() {
         false,
         [0.0, 0.0, 0.0, 1.0],
     )
-    .expect("block defn is cached");
+    .expect("block defn is cached")
+}
+
+#[test]
+fn block_nested_mtext_uses_its_style_font() {
+    // NB: on a host without Arial, `arial.ttf` resolves to the LFF stroke
+    // fallback, so this exercises the "something renders" path rather than the
+    // TTF canonicalisation specifically. The font-canonicalisation logic itself
+    // is covered by the unit tests in `text_support`; the colour-split and
+    // fill-pairing tests below cover the block-cache rendering changes.
+    let wires = expand_block_mtext(
+        "FERRAGAMO",
+        "arial.ttf",
+        Vector3::new(0.0, 0.0, 0.0),
+        Vector3::new(100.0, 50.0, 0.0),
+    );
 
     assert!(
         drawable_point_count(&wires) > 0,
@@ -58,7 +81,84 @@ fn block_nested_mtext_uses_its_style_font() {
     );
 
     assert!(
-        wires.iter().all(|w| w.points.is_empty() || w.fill_tris.is_empty()),
+        wires
+            .iter()
+            .all(|w| w.points.is_empty() || w.fill_tris.is_empty()),
         "outline and fill wires should be separate for correct GPU classification"
     );
 }
+
+#[test]
+fn block_nested_colour_split_mtext_keeps_per_wire_colour() {
+    // `\C1;` = ACI red, `\C2;` = ACI yellow → two colour bins. The block cache
+    // must keep them as separate per-wire colours; the fold-to-one-colour bug
+    // this PR fixes silently collapsed every segment to the first colour. Uses
+    // the builtin stroke font so the split is observable without a system TTF.
+    let wires = expand_block_mtext(
+        "\\C1;AAA\\C2;BBB",
+        "txt",
+        Vector3::new(0.0, 0.0, 0.0),
+        Vector3::new(100.0, 50.0, 0.0),
+    );
+
+    let mut colours: Vec<[u8; 3]> = wires
+        .iter()
+        .filter(|w| !w.points.is_empty() || !w.fill_tris.is_empty())
+        .map(|w| {
+            [
+                (w.color[0] * 255.0).round() as u8,
+                (w.color[1] * 255.0).round() as u8,
+                (w.color[2] * 255.0).round() as u8,
+            ]
+        })
+        .collect();
+    colours.sort();
+    colours.dedup();
+
+    assert!(
+        colours.len() >= 2,
+        "colour-split MTEXT in a block must keep ≥2 distinct wire colours, got {colours:?}"
+    );
+}
+
+#[test]
+fn block_nested_mtext_fill_tris_keep_paired_low_half_at_utm() {
+    // TTF glyph fills are host-dependent (builtin LFF stroke fonts produce
+    // none). When the host resolves a TTF and fills ARE produced, every fill
+    // wire must carry an index-paired `fill_tris_low`: emit_wire reconstructs
+    // `fill_tris[i] + fill_tris_low[i]`, so an unpaired wire panics (release
+    // too) and a dropped low half quantizes fills to ~0.5 m at UTM scale.
+    // The MTEXT sits at a UTM-scale coordinate so the low half is significant.
+    // "DejaVu Sans" is present on most Linux hosts; where no TTF resolves the
+    // fill set is empty and the assertions are skipped (gate below).
+    let wires = expand_block_mtext(
+        "FERRAGAMO",
+        "DejaVu Sans",
+        Vector3::new(500_000.0, 4_000_000.0, 0.0),
+        Vector3::new(0.0, 0.0, 0.0),
+    );
+
+    let fill_wires: Vec<&WireModel> = wires.iter().filter(|w| !w.fill_tris.is_empty()).collect();
+    if fill_wires.is_empty() {
+        eprintln!("no TTF fills resolvable on this host; skipping fill-pairing assertions");
+        return;
+    }
+
+    for w in &fill_wires {
+        assert_eq!(
+            w.fill_tris.len(),
+            w.fill_tris_low.len(),
+            "fill_tris and fill_tris_low must be index-paired for emit_wire"
+        );
+    }
+
+    let any_nonzero_low = fill_wires
+        .iter()
+        .flat_map(|w| w.fill_tris_low.iter())
+        .any(|p| p.iter().any(|&c| c != 0.0));
+    assert!(
+        any_nonzero_low,
+        "UTM-scale fills must keep a non-zero low half — a dropped fill_tris_low \
+         silently quantizes fill triangles to ~0.5 m"
+    );
+}