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, 28 insertions, 0 deletions

diff --git a/rust/crates/commands/src/preview_tracker.rs b/rust/crates/commands/src/preview_tracker.rs
new file mode 100644
index 00000000..4eb7484d
--- /dev/null
+++ b/rust/crates/commands/src/preview_tracker.rs
@@ -0,0 +1,28 @@
+#[derive(Clone, Debug, Default)]
+pub struct PreviewTracker<T> {
+    snapshot: Option<T>,
+}
+
+impl<T: Clone> PreviewTracker<T> {
+    pub fn new() -> Self {
+        Self { snapshot: None }
+    }
+
+    pub fn begin(&mut self, state: &T) {
+        if self.snapshot.is_none() {
+            self.snapshot = Some(state.clone());
+        }
+    }
+
+    pub fn is_active(&self) -> bool {
+        self.snapshot.is_some()
+    }
+
+    pub fn snapshot(&self) -> Option<&T> {
+        self.snapshot.as_ref()
+    }
+
+    pub fn end(&mut self) -> Option<T> {
+        self.snapshot.take()
+    }
+}