resolve_font previously returned &str, passing raw stems like "arial" verbatim to Face::resolve. System font lookup requires the canonical family name ("Arial"), so inline \f/\F overrides from block-nested MTEXT were silently unresolvable and fell through to no geometry.
Change resolve_font to return Cow<'a, str>:
- no override -> Borrowed(base), zero allocation
- LFF built-in -> Borrowed(font), zero allocation
- system font resolved -> Owned(canonical), one allocation
- unresolvable -> Borrowed(base), falls back to style font
All 4 call sites updated to &font_name (&Cow<str> derefs to &str).
Adds two unit tests covering the fallback and TTF-stem-to-family-name paths. Adds integration test verifying block-nested MTEXT produces geometry and correctly separates outline/fill wires.
06bb6b54ff