Review: short "d" falls back to the font ascent; tracking unit comment; Lottie hunk out

- first_ascent: a "d" shorter than 0.3 x the font ascent (a symbol or icon font) no
  longer puts box text's first baseline almost at the box top; the font ascent is used,
  as before the "d" rule (CodeRabbit on #1875). Test:
  a_short_d_falls_back_to_the_font_ascent.
- CharXf::track_before / track_after are pixels, not 1/1000 em (CodeRabbit).
- The Lottie export of style-run tracking is back to what it was: the pixel conversion
  was not checked against a player, so it is left for a separate, verified change.
- The Subtract clamp in Mode::combine is intended for every animator property: the
  selectors combine into one selection per character for the whole animator, as in
  After Effects; the read-back observed it through Tracking (now said in the code).
- Per-character 3D rotation notes that #1876 adds Mat4::rotation_xyz for the same order.

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

crates/render/src/text.rs

@@ -42,7 +42,8 @@ pub struct CharXf {
     pub stroke_hsb: [f64; 3],
     pub stroke_opacity: f64,
     pub stroke_width: f64,
-    /// Tracking (1/1000 em) added before / after the character.
+    /// Tracking (pixels: an animator's Tracking Amount, as After Effects) added before / after
+    /// the character.
     pub track_before: f64,
     pub track_after: f64,
     /// Line Anchor (percent) and the selection weight it was set with.
@@ -752,6 +753,9 @@ pub fn text_geom(ctx: &EvalCtx, layer: &Layer) -> Option<TextGeom> {
         };
         // Outline relative to the pivot.
         let local = kurbo::Affine::translate((o[0] - pivot[0], o[1] - pivot[1])) * outline;
+        // Rx · Ry · Rz (Z turns first): the order After Effects gives a 3D layer's X / Y / Z
+        // Rotation too. #1876 moves layers to it with a helper, `Mat4::rotation_xyz`; once both
+        // land, this should call that helper (one convention, one place).
         let char_m = Mat4::rotate_x(x.rotation_x)
             * Mat4::rotate_y(x.rotation_y)
             * Mat4::rotate_z(x.rotation)