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, 29 insertions, 0 deletions

diff --git a/rust/crates/audio/src/lib.rs b/rust/crates/audio/src/lib.rs
new file mode 100644
index 00000000..0bce5647
--- /dev/null
+++ b/rust/crates/audio/src/lib.rs
@@ -0,0 +1,29 @@
+mod buckets;
+mod clips;
+mod mix;
+
+pub use buckets::{
+    extract_peak_range, extract_rms_buckets, extract_rms_range, extract_rms_windowed,
+    sample_bucket_range,
+};
+pub use clips::{
+    AudibleClip, clamp_db, clamp_retime_rate, collect_audible_elements, db_to_linear,
+    source_time_at_clip_time,
+};
+pub use mix::{apply_mastering, mix_range, render_export_buffer};
+
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    #[error(
+        "element {element_id} requires pitch-preserving retime at rate {rate}, which is not supported"
+    )]
+    MaintainPitchUnsupported { element_id: String, rate: f64 },
+    #[error("invalid time range: end must be after start")]
+    InvalidTimeRange,
+    #[error("sample rate must be at least 1")]
+    InvalidSampleRate,
+    #[error("channel count must be at least 1")]
+    InvalidChannelCount,
+}
+
+pub type Result<T> = std::result::Result<T, Error>;