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)

crates/oaknode/src/lib.rs

@@ -0,0 +1,53 @@
+// 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/>.
+
+//! # oaknode — the node graph engine (Rust)
+//!
+//! Reimplements the C++ oaknode module behind its frozen C ABI
+//! (`include/node/*.h`). See README.md for the architectural mapping
+//! (inheritance → arena + trait objects, etc.).
+//!
+//! ## FFI discipline
+//!
+//! Identical to the oakplugin crate: every export goes through
+//! [`handle::guard*`], handles are opaque refcounted boxes, shared
+//! state behind `Mutex`.
+
+#![deny(unsafe_op_in_unsafe_fn)]
+#![warn(missing_docs)]
+
+pub mod block;
+pub mod bridge;
+pub mod colormanager;
+pub mod error;
+pub mod factory;
+pub mod ffi;
+pub mod folder;
+pub mod footage;
+pub mod graph;
+pub mod handle;
+pub mod id;
+pub mod input;
+pub mod keyframe;
+pub mod node;
+pub mod nodes;
+pub mod ops;
+pub mod project;
+pub mod sequence;
+pub mod serializer;
+pub mod track;
+pub mod traverser;
+pub mod value;