refactor: workspace layout — crates/, app at root, legacy C++ removed

Single mechanical restructure commit:
- root Cargo.toml = oakapp bin + workspace; one cargo build produces
  oakapp, oak-cli, oak-worker, liboakengine.dylib
- app/rust/src -> src/ (app at repo root, no rust/ nesting)
- src/<mod>/rust -> crates/oak<mod>; src/oakcore-rs -> crates/oakcore;
  src/bindings/oakotio -> crates/oakotio; src/engine/rust ->
  crates/oakengine (keeps cdylib+staticlib+rlib)
- public C headers include/<mod>/ -> crates/oakengine/include/<mod>/
- OFX SDK headers vendored into crates/oakplugin/ofx/ (HostSupport gone)
- legacy deleted: old src/ C++ modules, engine/, core/, ffmpeg_bridge/,
  app/ (Qt), cli/worker C++, root CMakeLists, third_party/KDDockWidgets
  submodule, otio-install, all build-* output (~40GB)
- oakstorage kept but excluded from the workspace (skeleton w/ todos);
  gpui excluded (own workspace)
- verified: cargo build green, cargo test --workspace 1845/0
  (with the documented OCIO_RS_* env override for the homebrew OCIO)

examples/screenshot.rs

@@ -0,0 +1,76 @@
+// 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();
+
+	// Draw a few frames so the layout settles: the node editor fits its graph
+	// once the canvas size is known and the viewers upload their first CPU
+	// frame, both of which happen on the frame after the initial render.
+	for _ in 0..4 {
+		cx.run_until_parked();
+		cx.update_window(handle, |_root, window, app| {
+			let _ = window.draw(app);
+		})?;
+	}
+	cx.run_until_parked();
+
+	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(())
+}