Issue #2052: merge main and limit export fallback to Thai

Keep the existing Inter and CJK export order and the UI font chain.
Defer Arabic export faces until joining and bidirectional ordering exist.
Restore main's headline documents and record remaining text-layout limits.

crates/engine/src/text.rs

@@ -1,11 +1,12 @@
 //! Text drawing in pure Rust, shared by the export watermark and the slideshow overlays: Inter
-//! SemiBold (bundled) first, then the craft-fonts non-Latin faces as fallbacks ([`faces`]); glyph outlines
+//! SemiBold (bundled) first, then the craft-fonts CJK and Thai faces as fallbacks ([`faces`]); glyph outlines
 //! from `ab_glyph`, vertical CJK cells shaped with `harfrust`. [`TextLayout`] lays a string out
 //! once (lines split at `\n`, kerning within a face) and reports its size, so callers can anchor
 //! it before drawing; drawing hands out per-pixel coverage, and the caller blends.
 //!
-//! Horizontal text has no contextual shaping or bidi reordering. Arabic glyphs remain isolated
-//! and in logical order. Scripts no face covers draw as Inter's missing-glyph box.
+//! Horizontal text has no contextual shaping or bidi reordering; Thai combining marks use
+//! per-character metrics rather than shaped positioning. Arabic faces are not offered. Characters
+//! no face covers draw as Inter's missing-glyph box.
 
 use ab_glyph::{Font, FontRef, Glyph, PxScale, ScaleFont, point};
 
@@ -16,15 +17,16 @@ pub static TEXT_FONT: &[u8] = include_bytes!("../../../assets/fonts/Inter-SemiBo
 pub const MAX_CHARS: usize = 4000;
 
 /// The text faces: Inter first, then the craft-fonts CJK faces (Mincho first, the watermark's
-/// serif look; then any other CJK face), then the UI's other non-Latin fallback faces. Drawn text
-/// has no UI language, so every bundled script is offered. Without craft-fonts that is Inter
-/// alone, and unsupported characters draw as Inter's missing-glyph box.
+/// serif look; then any other CJK face), then Thai faces, preferring Regular. Drawn text has no
+/// UI language, so every CJK script is offered. Other script faces stay out until their layout is
+/// supported. Without craft-fonts that is Inter alone, with missing-glyph boxes for unsupported text.
 pub fn faces(craft: &'static [crate::fonts::CraftFont]) -> Vec<FontRef<'static>> {
     let mut cjk: Vec<_> = crate::fonts::cjk(craft).collect();
     cjk.sort_by_key(|f| !f.is_mincho());
-    let others =
-        crate::fonts::ui_fallback(craft, "Latn", "Regular").into_iter().filter(|f| !f.scripts.iter().any(|s| crate::fonts::is_cjk_script(s)));
-    std::iter::once(TEXT_FONT).chain(cjk.into_iter().chain(others).map(|f| f.bytes)).filter_map(|bytes| FontRef::try_from_slice(bytes).ok()).collect()
+    let thai = crate::fonts::ui_fallback(craft, "Latn", "Regular")
+        .into_iter()
+        .filter(|f| f.covers("Thai") && !f.scripts.iter().any(|s| crate::fonts::is_cjk_script(s)));
+    std::iter::once(TEXT_FONT).chain(cjk.into_iter().chain(thai).map(|f| f.bytes)).filter_map(|bytes| FontRef::try_from_slice(bytes).ok()).collect()
 }
 
 /// Shape a whole grapheme in one cell. CJK uses vertical origins and forms; Latin stays upright.
@@ -249,8 +251,17 @@ mod tests {
     }
 
     #[test]
-    fn bundled_arabic_uses_its_glyphs() {
-        assert_script_glyphs("Arab", "مرحبا");
+    fn bundled_arabic_stays_out_of_export_fallback() {
+        if !CRAFT_FONTS.iter().any(|font| font.covers("Arab")) {
+            lightcraft_testkit::skip!(Fonts, "built without the Arabic face from craft-fonts");
+            return;
+        }
+        // UI names still use their existing Arabic face. Export needs joining and bidi first.
+        assert!(crate::fonts::ui_fallback(CRAFT_FONTS, "Latn", "Regular").iter().any(|font| font.covers("Arab")));
+        for vertical in [false, true] {
+            let layout = TextLayout::new("مرحبا", 40.0, vertical, CRAFT_FONTS).unwrap();
+            assert!(layout.glyphs.iter().all(|(face, glyph, _)| *face == 0 && glyph.id.0 == 0));
+        }
     }
 
     #[test]