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 <[email protected]>

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(),
                 });
             }
         }

src/entities/multileader.rs

@@ -1304,9 +1304,11 @@ impl MultiLeaderTess for MultiLeader {
             plinegen: true,
             vp_scissor: None,
             fill_tris: 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(),
         });
 
@@ -1596,9 +1598,11 @@ impl MultiLeaderTess for MultiLeader {
                         plinegen: true,
                         vp_scissor: None,
                         fill_tris: greek_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 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(),
                     });
                 }
@@ -1676,9 +1680,11 @@ impl MultiLeaderTess for MultiLeader {
                         plinegen: true,
                         vp_scissor: None,
                         fill_tris: text_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 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(),
                     });
                 }

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(),
             }
         };

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(

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