feat: GPUI desktop port foundation

- scene crate: full project/scene/element model, serde round-trip
  proven against the web SerializedProject shape
- transfer crate: .ocp zip container, cross-compat with web fflate
  archives proven in both directions
- storage crate: filesystem project store replacing IndexedDB,
  .ocp import/export
- media crate: ffmpeg-next 8 decoder (frames, audio, thumbnails,
  waveform summaries)
- playback crate: wall-clock anchored, frame-rounded playback clock
- gpu crate: native texture readback; compositor renders headless
  offscreen on Vulkan (spike test)
- flake devshell: rust toolchain, GPUI native deps, ffmpeg dev,
  bindgen clang args; GPUI hello-world builds
- port plan in notes/gpui-port.md

1 files changed, 44 insertions, 0 deletions

diff --git a/rust/crates/timeline/src/placement/main_track.rs b/rust/crates/timeline/src/placement/main_track.rs
new file mode 100644
index 00000000..01d91d4f
--- /dev/null
+++ b/rust/crates/timeline/src/placement/main_track.rs
@@ -0,0 +1,44 @@
+use scene::{Element, SceneTracks, Track};
+use time::MediaTime;
+
+pub const MAIN_TRACK_NAME: &str = "Main Track";
+
+pub fn get_earliest_main_track_element<'a>(
+    main_track: &'a Track,
+    exclude_element_id: Option<&str>,
+) -> Option<&'a Element> {
+    main_track
+        .elements()
+        .iter()
+        .filter(|element| exclude_element_id != Some(element.base().id.as_str()))
+        .reduce(|earliest, element| {
+            if element.base().start_time < earliest.base().start_time {
+                element
+            } else {
+                earliest
+            }
+        })
+}
+
+pub fn enforce_main_track_start(
+    tracks: &SceneTracks,
+    target_track_id: &str,
+    requested_start_time: MediaTime,
+    exclude_element_id: Option<&str>,
+) -> MediaTime {
+    if tracks.main.id() != target_track_id {
+        return requested_start_time;
+    }
+
+    let Some(earliest_element) =
+        get_earliest_main_track_element(&tracks.main, exclude_element_id)
+    else {
+        return MediaTime::ZERO;
+    };
+
+    if requested_start_time <= earliest_element.base().start_time {
+        return MediaTime::ZERO;
+    }
+
+    requested_start_time
+}