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/oakplugin/src/error.rs

@@ -0,0 +1,62 @@
+// 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/>.
+
+//! 错误码。与 `include/*/error.h` 逐字对应；项目统一 -MMCCCC 方案
+//! （模块号注册表见 include/common/error.h），跨模块透传不翻译。
+
+/// 成功。
+pub const OAKPLUGIN_OK: i32 = 0;
+/// 空句柄或非法参数。
+pub const OAKPLUGIN_E_INVALID: i32 = -90001;
+/// 当前状态不允许该调用（如未扫描就创建实例）。
+pub const OAKPLUGIN_E_STATE: i32 = -90002;
+/// 底层操作失败（OFX action 返回非 kOfxStatOK、插件入口拒绝等）。
+pub const OAKPLUGIN_E_FAILED: i32 = -90003;
+/// 索引越界 / 指定标识的插件不存在。
+pub const OAKPLUGIN_E_NOT_FOUND: i32 = -90004;
+/// 分配失败。
+pub const OAKPLUGIN_E_NOMEM: i32 = -90005;
+
+/// crate 内部统一的结果类型；FFI 层把它映射为上述 i32 码。
+pub type Result<T> = std::result::Result<T, Error>;
+
+/// crate 内部错误。`code()` 给出对外错误码。
+#[derive(Debug)]
+pub enum Error {
+	/// 空句柄或非法参数。
+	Invalid,
+	/// 状态不允许。
+	State,
+	/// 底层失败，附人类可读上下文（仅日志，不出界）。
+	Failed(String),
+	/// 未找到。
+	NotFound,
+	/// 分配失败。
+	NoMem,
+}
+
+impl Error {
+	/// 映射为 `include/plugin/error.h` 的错误码。
+	pub fn code(&self) -> i32 {
+		match self {
+			Error::Invalid => OAKPLUGIN_E_INVALID,
+			Error::State => OAKPLUGIN_E_STATE,
+			Error::Failed(_) => OAKPLUGIN_E_FAILED,
+			Error::NotFound => OAKPLUGIN_E_NOT_FOUND,
+			Error::NoMem => OAKPLUGIN_E_NOMEM,
+		}
+	}
+}