feat: oakengine facade, oaknode/oakrender impls, worker+CLI, app skeleton

- oaknode Rust crate: full implementation (core engine, sequence/
  track/block/footage, traverser, serializer, 43 node behaviors;
  493 tests green)
- oakrender Rust crate: full implementation incl. wgpu backend
  skeleton, ticket arena, worker pool (136 tests green; fixed
  lost-wakeup and ticket ordering races)
- src/facade/rust (oakfacade): 222 oakengine_* exports over the
  module C ABIs (61 tests green); worker_main + real POSIX shm
  frame-slot transport (SpscRingBuffer/FrameSlotPool, wire-compatible
  with engine/render/ipc)
- cli/rust + worker/rust binaries (29 + 29 tests green)
- oakotio: FCPXML import/export (49 tests green)
- oaktask: OTIO/FCPXML format dispatch (90 tests green)
- app/rust: gpui app skeleton — dock panels (viewers/timeline/
  explorer/inspector/node editor), transport, olive themes,
  i18n (en/zh), 37 tests green
- gpui submodule: menu checkmarks, dock ratios, vertical meter,
  CPU-frame viewer surface, drop-frame timecode

app/rust/examples/screenshot.rs

@@ -0,0 +1,72 @@
+// Oak Video Editor - Non-Linear Video Editor
+// Copyright (C) 2026 Oak Team
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Offscreen screenshot capture for the app window.
+//!
+//! Renders the full [`OakApp`] shell at 1600×900 (2× = 3200×1866 px) in an
+//! offscreen macOS window and writes the PNG to
+//! `app/rust/docs/screenshot-window.png`, using the same
+//! [`VisualTestAppContext`] machinery the gpui visual tests use. The window
+//! is created at `(-10000, -10000)` so nothing flickers on screen.
+//!
+//! Run it on the macOS main thread (examples run on the main thread, unlike
+//! `#[test]` harness threads):
+//!
+//! ```text
+//! cargo run --example screenshot               # 1600×900 → docs/screenshot-window.png
+//! cargo run --example screenshot -- 1100 900   # any size (still overwrites the same file)
+//! ```
+
+use gpui::{px, size, AnyWindowHandle, AppContext, Result, VisualTestAppContext};
+use gpui_platform::current_platform;
+use oakapp::app::OakApp;
+
+const DEFAULT_WIDTH: f32 = 1600.0;
+const DEFAULT_HEIGHT: f32 = 900.0;
+const OUT: &str = concat!(env!("CARGO_MANIFEST_DIR"), "/docs/screenshot-window.png");
+
+fn main() -> Result<()> {
+	let args: Vec<String> = std::env::args().skip(1).collect();
+	let width = args
+		.first()
+		.and_then(|s| s.parse().ok())
+		.unwrap_or(DEFAULT_WIDTH);
+	let height = args
+		.get(1)
+		.and_then(|s| s.parse().ok())
+		.unwrap_or(DEFAULT_HEIGHT);
+
+	let mut cx = VisualTestAppContext::new(current_platform(false));
+	cx.update(|app| app.init_colors());
+
+	let window = cx.open_offscreen_window(size(px(width), px(height)), |window, cx| {
+		cx.new(|cx| OakApp::new(window, cx))
+	})?;
+	let handle: AnyWindowHandle = window.into();
+
+	// Let the platform settle, then draw one full frame into the rendered
+	// scene so `render_to_image` has something to capture.
+	cx.run_until_parked();
+	cx.update_window(handle, |_root, window, app| {
+		let _ = window.draw(app);
+	})?;
+
+	let image = cx.capture_screenshot(handle)?;
+	std::fs::create_dir_all(std::path::Path::new(OUT).parent().unwrap())?;
+	image.save(OUT)?;
+	println!("wrote {OUT} ({}×{})", image.width(), image.height());
+	Ok(())
+}