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/entities/dimension.rs

@@ -1261,9 +1261,11 @@ fn tessellate_dimension_inner(
         plinegen: true,
         vp_scissor: None,
         fill_tris: geom.arrow_fill,
-        // FIXME: fill_tris_low left empty — needs double-single split to match
-        // fill_tris. Any geometry from this path inside a block definition will
-        // trip the debug_assert_eq! in emit_wire (block_cache.rs:1397).
+        // fill_tris_low intentionally empty: this fill renders on the top-level
+        // path, where consumers (face3d_gpu, xclip) treat a short low half as
+        // all-zero, so it draws at f32 precision (sub-metre error at UTM scale)
+        // — not a crash. Follow-up: double-single-split via points_to_ds to
+        // match emit_wire's paired fill path.
         fill_tris_low: Vec::new(),
     });
 
@@ -1296,11 +1298,13 @@ fn tessellate_dimension_inner(
                     aabb: WireModel::UNBOUNDED_AABB,
                     plinegen: true,
                     vp_scissor: None,
-        fill_tris: rect,
-        // FIXME: fill_tris_low left empty — needs double-single split to match
-        // fill_tris. Any geometry from this path inside a block definition will
-        // trip the debug_assert_eq! in emit_wire (block_cache.rs:1397).
-        fill_tris_low: Vec::new(),
+                    fill_tris: rect,
+                    // fill_tris_low intentionally empty: this fill renders on the
+                    // top-level path, where consumers (face3d_gpu, xclip) treat a
+                    // short low half as all-zero, so it draws at f32 precision
+                    // (sub-metre error at UTM scale) — not a crash. Follow-up:
+                    // double-single-split via points_to_ds to match emit_wire.
+                    fill_tris_low: Vec::new(),
                 });
             }
         }