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/app/view.rs

@@ -1975,7 +1975,7 @@ const MTEXT_FONTS: [&str; 10] = [
     "ItalicC",
     "ScriptS",
     "GothGBT",
-    "Cursive",
+    "RomanC",
 ];
 /// (label, ACI). 256 = ByLayer.
 const MTEXT_COLORS: [(&str, u16); 8] = [