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, 81 insertions, 0 deletions

diff --git a/rust/crates/timeline/src/placement/insert_index.rs b/rust/crates/timeline/src/placement/insert_index.rs
new file mode 100644
index 00000000..b0feaec6
--- /dev/null
+++ b/rust/crates/timeline/src/placement/insert_index.rs
@@ -0,0 +1,81 @@
+use scene::SceneTracks;
+
+use super::InsertDirection;
+use crate::types::TrackType;
+
+pub fn get_default_insert_index_for_track(tracks: &SceneTracks, track_type: TrackType) -> usize {
+    if track_type == TrackType::Audio {
+        return tracks.overlay.len() + 1 + tracks.audio.len();
+    }
+
+    if track_type == TrackType::Effect {
+        return 0;
+    }
+
+    tracks.overlay.len()
+}
+
+pub fn get_highest_insert_index_for_track(tracks: &SceneTracks, track_type: TrackType) -> usize {
+    if track_type == TrackType::Audio {
+        return tracks.overlay.len() + 1;
+    }
+
+    0
+}
+
+pub struct PreferredNewTrackPlacement {
+    pub insert_index: usize,
+    pub insert_position: Option<InsertDirection>,
+}
+
+pub fn resolve_preferred_new_track_placement(
+    tracks: &SceneTracks,
+    track_type: TrackType,
+    preferred_index: usize,
+    direction: InsertDirection,
+) -> PreferredNewTrackPlacement {
+    let track_count = tracks.overlay.len() + 1 + tracks.audio.len();
+    if track_count == 0 {
+        return PreferredNewTrackPlacement {
+            insert_index: 0,
+            insert_position: (track_type == TrackType::Audio)
+                .then_some(InsertDirection::Below),
+        };
+    }
+
+    let safe_preferred_index = preferred_index.min(track_count - 1);
+    let main_track_index = tracks.overlay.len();
+
+    if track_type == TrackType::Audio {
+        if safe_preferred_index <= main_track_index {
+            return PreferredNewTrackPlacement {
+                insert_index: main_track_index + 1,
+                insert_position: Some(InsertDirection::Below),
+            };
+        }
+
+        return PreferredNewTrackPlacement {
+            insert_index: match direction {
+                InsertDirection::Above => safe_preferred_index,
+                InsertDirection::Below => safe_preferred_index + 1,
+            },
+            insert_position: Some(direction),
+        };
+    }
+
+    let insert_index = match direction {
+        InsertDirection::Above => safe_preferred_index,
+        InsertDirection::Below => safe_preferred_index + 1,
+    };
+    if insert_index > main_track_index {
+        return PreferredNewTrackPlacement {
+            insert_index: main_track_index,
+            insert_position: Some(InsertDirection::Above),
+        };
+    }
+
+    PreferredNewTrackPlacement {
+        insert_index,
+        insert_position: Some(direction),
+    }
+}