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, 34 insertions, 0 deletions

diff --git a/rust/crates/timeline/src/placement/overlap.rs b/rust/crates/timeline/src/placement/overlap.rs
new file mode 100644
index 00000000..98055e9a
--- /dev/null
+++ b/rust/crates/timeline/src/placement/overlap.rs
@@ -0,0 +1,34 @@
+use scene::Element;
+use time::MediaTime;
+
+use super::PlacementTimeSpan;
+
+fn would_element_overlap(
+    elements: &[Element],
+    start_time: MediaTime,
+    end_time: MediaTime,
+    exclude_element_id: Option<&str>,
+) -> bool {
+    elements.iter().any(|element| {
+        if exclude_element_id == Some(element.base().id.as_str()) {
+            return false;
+        }
+
+        let element_end = element.base().start_time + element.base().duration;
+        start_time < element_end && end_time > element.base().start_time
+    })
+}
+
+pub fn can_place_time_spans_on_track(
+    elements: &[Element],
+    time_spans: &[PlacementTimeSpan],
+) -> bool {
+    time_spans.iter().all(|span| {
+        !would_element_overlap(
+            elements,
+            span.start_time,
+            span.start_time + span.duration,
+            span.exclude_element_id.as_deref(),
+        )
+    })
+}