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)