feat: port domain crates to Rust (session 1 complete)

All green: cargo test --workspace (~300 tests, 0 failures).
- timeline (45): placement, resize/ripple, split, snap, retime,
  audio separation, scene helpers
- animation (73): bezier solver, channel sampling, keyframe ops,
  split/clamp/clone, param definitions
- commands (9): Command trait, BatchCommand, history, 19 concrete
  commands, freeze-frame port 1:1
- audio (22): audible collection, range mixing, mastering limiter,
  rms/peak buckets
- text (19): cosmic-text layout/measure/raster, backgrounds,
  decorations
- graphics (52): full CSS gradient parser, tiny-skia raster,
  4 shape definitions
- effect-defs (16): blur definition, registry, pass resolution
- mask-defs (19): all 9 mask renderers via tiny-skia
- renderer (6): node model + scene builder port
- desktop: GPUI projects screen wired to ProjectStore
- scene: factory (web defaults: v32, Main Track, 1920x1080, 30fps)

1 files changed, 111 insertions, 0 deletions

diff --git a/rust/crates/timeline/tests/retime.rs b/rust/crates/timeline/tests/retime.rs
new file mode 100644
index 00000000..b495fec6
--- /dev/null
+++ b/rust/crates/timeline/tests/retime.rs
@@ -0,0 +1,111 @@
+use scene::RetimeConfig;
+use timeline::retime::{
+    clamp_retime_rate, get_clip_time_at_source_time, get_effective_rate_at,
+    get_source_span_at_clip_time, get_source_time_at_clip_time,
+    get_timeline_duration_for_source_span, split_retime_at_clip_time,
+};
+
+const TWO_X: RetimeConfig = RetimeConfig {
+    rate: 2.0,
+    maintain_pitch: None,
+};
+const HALF_X: RetimeConfig = RetimeConfig {
+    rate: 0.5,
+    maintain_pitch: None,
+};
+
+#[test]
+fn maps_clip_time_to_source_time_at_2x_speed() {
+    assert_eq!(get_source_time_at_clip_time(5.0, Some(&TWO_X)), 10.0);
+}
+
+#[test]
+fn maps_clip_time_to_source_time_at_half_speed() {
+    assert_eq!(get_source_time_at_clip_time(4.0, Some(&HALF_X)), 2.0);
+}
+
+#[test]
+fn returns_clip_time_unchanged_when_no_retime() {
+    assert_eq!(get_source_time_at_clip_time(7.0, None), 7.0);
+}
+
+#[test]
+fn inverts_source_time_back_to_clip_time_at_2x_speed() {
+    assert_eq!(get_clip_time_at_source_time(10.0, Some(&TWO_X)), 5.0);
+}
+
+#[test]
+fn returns_effective_rate() {
+    assert_eq!(get_effective_rate_at(Some(&TWO_X)), 2.0);
+    assert_eq!(get_effective_rate_at(None), 1.0);
+}
+
+#[test]
+fn derives_timeline_duration_for_a_visible_source_span() {
+    assert_eq!(
+        get_timeline_duration_for_source_span(10.0, Some(&TWO_X)),
+        5.0
+    );
+    assert_eq!(
+        get_timeline_duration_for_source_span(10.0, Some(&HALF_X)),
+        20.0
+    );
+}
+
+#[test]
+fn clamps_invalid_rates_to_one() {
+    let zero = RetimeConfig {
+        rate: 0.0,
+        maintain_pitch: None,
+    };
+    let negative = RetimeConfig {
+        rate: -1.0,
+        maintain_pitch: None,
+    };
+    assert_eq!(get_source_time_at_clip_time(5.0, Some(&zero)), 5.0);
+    assert_eq!(get_source_time_at_clip_time(5.0, Some(&negative)), 5.0);
+    assert_eq!(clamp_retime_rate(f64::NAN), 1.0);
+    assert_eq!(clamp_retime_rate(f64::INFINITY), 1.0);
+}
+
+#[test]
+fn caps_retime_rates_above_5x() {
+    let hundred = RetimeConfig {
+        rate: 100.0,
+        maintain_pitch: None,
+    };
+    assert_eq!(get_source_time_at_clip_time(5.0, Some(&hundred)), 25.0);
+    assert_eq!(
+        get_timeline_duration_for_source_span(10.0, Some(&hundred)),
+        2.0
+    );
+}
+
+#[test]
+fn measures_source_span_at_a_clip_time() {
+    assert_eq!(get_source_span_at_clip_time(5.0, Some(&TWO_X)), 10.0);
+}
+
+#[test]
+fn returns_zero_for_non_positive_clip_time() {
+    assert_eq!(get_source_span_at_clip_time(0.0, None), 0.0);
+    assert_eq!(get_source_span_at_clip_time(-1.0, None), 0.0);
+}
+
+#[test]
+fn passes_the_same_retime_to_both_halves_when_splitting() {
+    let retime = RetimeConfig {
+        rate: 1.5,
+        maintain_pitch: None,
+    };
+    let (left, right) = split_retime_at_clip_time(Some(&retime));
+    assert_eq!(left, Some(retime));
+    assert_eq!(right, Some(retime));
+}
+
+#[test]
+fn returns_none_on_both_sides_when_no_retime() {
+    let (left, right) = split_retime_at_clip_time(None);
+    assert_eq!(left, None);
+    assert_eq!(right, None);
+}