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>

src/scene/cache/block_cache.rs

@@ -1138,6 +1138,17 @@ fn expand_defn(
                             }
                         } else {
                             if h_px < 1.0 {
+                                // Colour-split MTEXT: only the first outline
+                                // wire from a source entity emits the baseline
+                                // substitute. Siblings share the same
+                                // text_obb_local and would stack identical
+                                // overlapping lines (last colour wins) — the
+                                // same dedup the greek branch does, one tier
+                                // down.
+                                if lw.text_obb_local == last_lod_obb {
+                                    continue;
+                                }
+                                last_lod_obb = lw.text_obb_local;
                                 emit_text_baseline(lw, accum_xform, ctx, out, wpp);
                                 continue;
                             }
@@ -1431,7 +1442,6 @@ fn emit_wire(
         entry.fill_tris.push([hx, hy, hz]);
         entry.fill_tris_low.push([lx, ly, lz]);
     }
-
 }
 
 fn transform_tangent(