fix(text): keep inline \C colours through block colour inheritance (SDF)

Ports PR #301 (by Kevin Griffin) to the SDF text renderer. Block-nested MTEXT
with inline colour codes (\C1;, \c…) collapsed to a single colour: the block
cache applies ByBlock / layer-0 colour inheritance per entity, so at emit time
every segment's colour was replaced with the inherited colour — including
segments carrying an explicit inline override.

- block_cache tessellate_sub_local: gate ByBlock / layer-0 colour inheritance
  on `wire_on_base_color` (a wire whose colour differs from the entity base
  carries an explicit override and keeps it) — #301's fix, still correct for
  the remaining stroke wires.
- block_cache emit_wire: the per-vertex analogue for SDF text — a glyph whose
  colour equals the wire's base inherits the resolved colour; a glyph with an
  inline \C / \c override keeps it. (Previously every block glyph was recoloured
  with final_color, collapsing colour-split MTEXT.)
- tests/text_font_rendering.rs: restore the block colour-split regression test
  (it had stopped compiling on main when expand_insert gained the InheritStyle
  param) and adapt it — and drawable_point_count — to SDF text_verts, since
  block MTEXT now renders as glyph quads, not colour-split stroke wires.

\C (ACI) / \c (true-colour) are parsed by acadrust (mtext_format
SpanProperties.color) → OCS InlineColor → the glyph's per-vertex colour.

Co-Authored-By: Kevin Griffin <117586586+KevinGriffin-new@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/cache/block_cache.rs

@@ -447,6 +447,13 @@ fn tessellate_sub_local(
                 .chain(wire.text_verts.iter().map(|v| v.pos)),
         );
         let is_fill_only = wire.points.is_empty() && !wire.fill_tris.is_empty();
+        // A wire whose colour differs from the entity's resolved base colour
+        // carries an explicit per-segment override (e.g. an MTEXT `\C1;` inline
+        // colour). ByBlock / layer-0 inheritance applies only to wires still on
+        // the base colour — folding an explicit segment into the inherited
+        // colour would collapse colour-split geometry to one colour. (PR #301,
+        // Kevin Griffin — extended to SDF text per-vertex in emit_wire.)
+        let wire_on_base_color = wire.color == sub_color;
 
         result.push(LocalWire {
             points: wire.points,
@@ -464,10 +471,10 @@ fn tessellate_sub_local(
             line_weight_px: lw_px,
             plinegen: wire.plinegen,
             is_fill_only,
-            color_is_byblock,
+            color_is_byblock: color_is_byblock && wire_on_base_color,
             lt_is_byblock,
             lw_is_byblock,
-            color_l0,
+            color_l0: color_l0 && wire_on_base_color,
             lt_l0,
             lw_l0,
             aabb_local,
@@ -1243,11 +1250,22 @@ fn emit_wire(
         if hy > entry.max_y {
             entry.max_y = hy;
         }
+        // Base glyphs inherit the resolved (ByBlock / layer-0) colour; a glyph
+        // carrying an inline `\C` / `\c` override — colour differs from the
+        // wire's base — keeps it, so block-nested colour-split MTEXT stays
+        // multi-colour. Per-vertex analogue of PR #301's wire-level gate.
+        let rgb = if [tv.color[0], tv.color[1], tv.color[2]]
+            == [lw.color[0], lw.color[1], lw.color[2]]
+        {
+            [final_color[0], final_color[1], final_color[2]]
+        } else {
+            [tv.color[0], tv.color[1], tv.color[2]]
+        };
         entry.text_verts.push(crate::scene::pipeline::text_gpu::TextVertex {
             pos: [hx, hy, hz],
             pos_low: [lx, ly, lz],
             uv: tv.uv,
-            color: [final_color[0], final_color[1], final_color[2], tv.color[3]],
+            color: [rgb[0], rgb[1], rgb[2], tv.color[3]],
             draw_depth: tv.draw_depth,
         });
     }