feat: sketch text takes a font, an installed family or a font file (#516)

sketch.text and sketch.edit_text take an optional font: an installed family or a .ttf/.otf/.ttc file. Edit Text gets a Font field. The font is kept with the text under format 16; no fonts are bundled, and the font search is bounded.

Fixes #513

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/format.rs

@@ -55,6 +55,11 @@
 //!   round or rectangular). Version 15 designs need no transformation (an absent shape is the
 //!   notch they always had); the version stamp lets older readers request an update instead of
 //!   silently building notches where a design asks for another shape.
+//!   Under the same version, sketch text can be set in a font other than the built-in one
+//!   (`font` in the text link's source: an installed family or a font file); a text without
+//!   `font` is in the built-in font, as it always was.
+//!   A Move feature can also keep its source bodies and transform a copy (`copy: true`); a
+//!   move without it saves as before.
 
 use serde_json::Value;