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, 118 insertions, 0 deletions

diff --git a/rust/crates/commands/tests/common/mod.rs b/rust/crates/commands/tests/common/mod.rs
new file mode 100644
index 00000000..4cf45a9a
--- /dev/null
+++ b/rust/crates/commands/tests/common/mod.rs
@@ -0,0 +1,118 @@
+#![allow(dead_code)]
+
+use scene::{
+    Background, BaseTimelineElement, CanvasSize, Element, ImageElement, ParamValue,
+    ParamValues, Project, ProjectMetadata, ProjectSettings, Scene, VideoElement,
+};
+use time::{FrameRate, MediaTime};
+
+pub const SPLIT_SECONDS: f64 = 4.0;
+pub const FREEZE_SECONDS: f64 = 3.0;
+pub const VIDEO_SECONDS: f64 = 10.0;
+
+pub fn seconds(value: f64) -> MediaTime {
+    MediaTime::from_seconds_f64(value).expect("finite seconds")
+}
+
+pub fn default_params() -> ParamValues {
+    ParamValues::from([
+        ("transform.positionX".to_string(), ParamValue::Number(0.0)),
+        ("transform.positionY".to_string(), ParamValue::Number(0.0)),
+        ("transform.scaleX".to_string(), ParamValue::Number(1.0)),
+        ("transform.scaleY".to_string(), ParamValue::Number(1.0)),
+        ("transform.rotate".to_string(), ParamValue::Number(0.0)),
+        ("opacity".to_string(), ParamValue::Number(1.0)),
+    ])
+}
+
+pub fn base_element(id: &str, name: &str, duration: MediaTime) -> BaseTimelineElement {
+    BaseTimelineElement {
+        id: id.to_string(),
+        name: name.to_string(),
+        duration,
+        start_time: MediaTime::ZERO,
+        trim_start: MediaTime::ZERO,
+        trim_end: MediaTime::ZERO,
+        source_duration: None,
+        animations: None,
+        params: default_params(),
+    }
+}
+
+pub fn build_video_element() -> Element {
+    Element::Video(VideoElement {
+        base: base_element("video-1", "video-1", seconds(VIDEO_SECONDS)),
+        media_id: "media-video-1".to_string(),
+        is_source_audio_enabled: None,
+        hidden: None,
+        retime: None,
+        effects: None,
+        masks: None,
+    })
+}
+
+pub fn build_freeze_image_element() -> Element {
+    let mut base = base_element("freeze-template", "Freeze frame", seconds(FREEZE_SECONDS));
+    base.start_time = seconds(SPLIT_SECONDS);
+    Element::Image(ImageElement {
+        base,
+        media_id: "media-freeze-1".to_string(),
+        hidden: None,
+        effects: None,
+        masks: None,
+    })
+}
+
+pub fn active_scene(project: &Project) -> &Scene {
+    project
+        .scenes
+        .iter()
+        .find(|scene| scene.id == project.current_scene_id)
+        .expect("active scene")
+}
+
+pub fn seed_project_with_video() -> (Project, String) {
+    let now = chrono::DateTime::parse_from_rfc3339("2026-01-01T00:00:00Z")
+        .expect("valid date")
+        .to_utc();
+
+    let mut scene: Scene = timeline::scenes::build_default_scene(
+        "scene-1".to_string(),
+        "main-track-1".to_string(),
+        "Main scene".to_string(),
+        true,
+        now,
+    );
+    scene.tracks.main.elements_mut().push(build_video_element());
+    let track_id = scene.tracks.main.id().to_string();
+
+    let project = Project {
+        metadata: ProjectMetadata {
+            id: "project-1".to_string(),
+            name: "Test project".to_string(),
+            thumbnail: None,
+            duration: seconds(VIDEO_SECONDS),
+            created_at: now,
+            updated_at: now,
+        },
+        scenes: vec![scene],
+        current_scene_id: "scene-1".to_string(),
+        settings: ProjectSettings {
+            fps: FrameRate::new(30, 1),
+            canvas_size: CanvasSize {
+                width: 1920,
+                height: 1080,
+            },
+            canvas_size_mode: None,
+            last_custom_canvas_size: None,
+            original_canvas_size: None,
+            background: Background::Color {
+                color: "#000000".to_string(),
+            },
+        },
+        version: 1,
+        timeline_view_state: None,
+    };
+
+    (project, track_id)
+}