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, 104 insertions, 0 deletions

diff --git a/rust/crates/timeline/src/placement/mod.rs b/rust/crates/timeline/src/placement/mod.rs
new file mode 100644
index 00000000..8ad5710e
--- /dev/null
+++ b/rust/crates/timeline/src/placement/mod.rs
@@ -0,0 +1,104 @@
+use scene::{Element, SceneTracks, Track};
+use time::MediaTime;
+
+use crate::types::{ElementType, TrackType};
+
+pub mod apply;
+pub mod compatibility;
+pub mod insert_index;
+pub mod main_track;
+pub mod overlap;
+pub mod resolve;
+pub mod track_factory;
+
+pub use apply::{PlacementApplied, apply_placement};
+pub use compatibility::{
+    can_element_go_on_track, get_track_type_for_element_type, validate_element_track_compatibility,
+};
+pub use insert_index::{
+    get_default_insert_index_for_track, get_highest_insert_index_for_track,
+    resolve_preferred_new_track_placement,
+};
+pub use main_track::{MAIN_TRACK_NAME, enforce_main_track_start, get_earliest_main_track_element};
+pub use overlap::can_place_time_spans_on_track;
+pub use resolve::resolve_track_placement;
+pub use track_factory::build_empty_track;
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct PlacementTimeSpan {
+    pub start_time: MediaTime,
+    pub duration: MediaTime,
+    pub exclude_element_id: Option<String>,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum PlacementSubject {
+    ElementType(ElementType),
+    TrackType(TrackType),
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum InsertDirection {
+    Above,
+    Below,
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum VerticalDragDirection {
+    Up,
+    Down,
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum NewTrackPosition {
+    Highest,
+    Default,
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum PlacementStrategy {
+    Explicit {
+        track_id: String,
+    },
+    FirstAvailable,
+    PreferIndex {
+        track_index: usize,
+        hover_direction: InsertDirection,
+        vertical_drag_direction: Option<VerticalDragDirection>,
+        create_new_track_only: bool,
+    },
+    AboveSource {
+        source_track_index: usize,
+    },
+    AlwaysNew {
+        position: NewTrackPosition,
+    },
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum PlacementResult {
+    ExistingTrack {
+        track_id: String,
+        track_index: usize,
+        track_type: TrackType,
+        adjusted_start_time: Option<MediaTime>,
+    },
+    NewTrack {
+        track_type: TrackType,
+        insert_index: usize,
+        insert_position: Option<InsertDirection>,
+    },
+}
+
+pub fn ordered_tracks(tracks: &SceneTracks) -> Vec<&Track> {
+    tracks
+        .overlay
+        .iter()
+        .chain(std::iter::once(&tracks.main))
+        .chain(tracks.audio.iter())
+        .collect()
+}
+
+pub fn track_elements(track: &Track) -> &[Element] {
+    track.elements()
+}