feat: add shared Rust crates for cross-platform time utilities

Introduce `bridge` proc-macro crate with `#[export]` attribute that conditionally applies `wasm_bindgen` for WASM builds, and `time` crate porting all timecode/frame utilities from TypeScript to Rust.

rust/README.md

@@ -0,0 +1,35 @@
+# rust/
+
+Shared Rust crates that power OpenCut across platforms (web via WASM, desktop natively).
+
+## Adding a new crate
+
+1. Create it under `rust/crates/`
+2. Add `bridge` as a dependency
+3. Annotate public functions with `#[export]`
+
+## How `#[export]` works
+
+```rust
+use bridge::export;
+
+#[export]
+pub fn round_to_frame(time: f64, fps: f64) -> f64 {
+    (time * fps).round() / fps
+}
+```
+
+Without the `wasm` feature, the macro is a no-op. With `--features wasm`, it expands to:
+
+```rust
+#[wasm_bindgen(js_name = "roundToFrame")]
+pub fn round_to_frame(time: f64, fps: f64) -> f64 { ... }
+```
+
+Desktop uses the crates directly as Cargo dependencies.
+
+## Testing
+
+```bash
+cargo test -p <crate>
+```