feat(fonts): system TrueType support with shaping and fallback

Add a TrueType/OpenType render path alongside the LFF stroke engine so
users can pick fonts installed on their machine for text and MText.

- Discover system families via fontdb (sysfont) and list them in the
  text-style picker.
- Extract glyph outlines with ttf-parser, flattened into wire strokes
  normalized to the same 9-unit cap-height space as LFF glyphs.
- Shape runs with cosmic-text (ligatures, Arabic joining, kerning, bidi)
  plus automatic font fallback: a glyph missing from the chosen family
  is taken from another installed font.
- Unify both sources behind a single Face type; a shared run tokenizer
  drives the stroke (per-glyph) and shaped (TTF) renderers identically.
- Stroke fonts now fall back to a system TTF (cosmic-text picks it) for
  characters no LFF font covers, replacing the unicode/iso3098 chain.

Also trim the embedded LFF set from 47 to 26, keeping the AutoCAD-SHX
equivalents and dropping LibreCAD-only fonts now covered by TrueType.

LFF builtins keep priority over same-named system fonts so DXF text
renders identically across machines.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/mod.rs

@@ -4,6 +4,7 @@ mod camera;
 pub mod complex_lt;
 pub mod lff;
 pub mod dispatch;
+pub mod font_face;
 pub mod grip;
 pub mod hatch_model;
 pub mod hatch_patterns;
@@ -28,6 +29,8 @@ mod selection;
 pub mod acis_to_truck;
 pub mod solid3d_tess;
 pub mod spline_tess;
+pub mod sysfont;
+pub mod ttf_glyph;
 pub mod tess_util;
 pub mod tessellate;
 pub mod transform;