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/text/sysfont.rs

@@ -6,7 +6,8 @@
 // TTF glyph engine). LFF stroke fonts stay separate — this is purely the
 // TrueType side of the renderer.
 
-use std::sync::OnceLock;
+use std::collections::HashMap;
+use std::sync::{Mutex, OnceLock};
 
 struct SysFonts {
     db: fontdb::Database,
@@ -38,7 +39,27 @@ pub fn families() -> &'static [String] {
 }
 
 /// Resolve a requested family name to the canonical installed system family name (with exact case).
+///
+/// Memoised process-wide: `resolve_font` calls this once per word on the MTEXT
+/// measure hot path for inline-`\f` TTF runs, and `Face::resolve` re-runs it
+/// immediately after — the underlying fontdb query plus linear family scans are
+/// not free. The cache keys on the raw request string; results are stable for
+/// the process lifetime (the font DB is loaded once via `OnceLock`).
 pub fn canonical_family_name(family: &str) -> Option<String> {
+    static CACHE: OnceLock<Mutex<HashMap<String, Option<String>>>> = OnceLock::new();
+    let cache = CACHE.get_or_init(|| Mutex::new(HashMap::new()));
+    if let Some(hit) = cache.lock().unwrap().get(family) {
+        return hit.clone();
+    }
+    let resolved = canonical_family_name_uncached(family);
+    cache
+        .lock()
+        .unwrap()
+        .insert(family.to_string(), resolved.clone());
+    resolved
+}
+
+fn canonical_family_name_uncached(family: &str) -> Option<String> {
     let db = &fonts().db;
     
     // 1. Try exact match first