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/engine/src/view_display_tests.rs

@@ -25,7 +25,7 @@ fn sampler_keeps_style_precedence_hidden_references_text_and_disjoint_parts() {
     let text = sk
         .add_link(
             LinkKind::Text,
-            LinkSource::Text { text: "A".into(), at: Vec2::ZERO, height: 2., angle: 0. },
+            LinkSource::Text { text: "A".into(), at: Vec2::ZERO, height: 2., angle: 0., font: None },
             &[LinkGeom::Line(Vec2::new(0., 7.), Vec2::new(2., 7.))],
         )
         .unwrap();