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/table.rs

@@ -755,9 +755,11 @@ pub fn tessellate_table(
                 plinegen: true,
                 vp_scissor: None,
                 fill_tris,
-                // 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.
                 fill_tris_low: Vec::new(),
             }
         };