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/oakengine/include/common/error.h

@@ -0,0 +1,66 @@
+/***
+
+  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/>.
+
+***/
+
+#ifndef OAK_EDITOR_ERROR_H
+#define OAK_EDITOR_ERROR_H
+
+/**
+ * @brief Status and error codes shared by all oakcommon C API families.
+ *
+ * Return-code convention (mirrors engine/include/oakengine/init.h):
+ * 0 (OAKCOMMON_OK) on success, a negative OAKCOMMON_E_* error code on
+ * failure. String getters return the required buffer size in bytes
+ * (including the terminating NUL) as a non-negative value instead.
+ *
+ * Project-wide error code scheme (-MMCCCC, 2026-08):
+ * every module's error codes are negative integers of the form
+ * -(MM * 10000 + CCCC), where MM is the module number from the registry
+ * below and CCCC is a module-local code. The first module-local codes
+ * are reserved and identical across modules: 0001 INVALID, 0002 STATE,
+ * 0003 FAILED, 0004 NOT_FOUND, 0005 NOMEM, 0006 CANCELLED.
+ *
+ * An error code crossing a module boundary is passed through
+ * UNTRANSLATED — the numeric module prefix preserves provenance
+ * (e.g. -30004 is oaknode's NOT_FOUND no matter which module reports it
+ * to the caller).
+ *
+ * Module number registry (only ever appended to; numbers are frozen):
+ */
+#define OAK_ERROR_MODULE_COMMON 1   /**< oakcommon */
+#define OAK_ERROR_MODULE_UNDO 2     /**< oakundo */
+#define OAK_ERROR_MODULE_NODE 3     /**< oaknode */
+#define OAK_ERROR_MODULE_TIMELINE 4 /**< oaktimeline */
+#define OAK_ERROR_MODULE_CODEC 5    /**< oakcodec */
+#define OAK_ERROR_MODULE_AUDIO 6    /**< oakaudio */
+#define OAK_ERROR_MODULE_RENDER 7   /**< oakrender */
+#define OAK_ERROR_MODULE_TASK 8     /**< oaktask */
+#define OAK_ERROR_MODULE_PLUGIN 9   /**< oakplugin */
+#define OAK_ERROR_MODULE_STORAGE 10 /**< oakstorage (reserved) */
+
+#define OAKCOMMON_OK 0 /**< Success. */
+#define OAKCOMMON_E_INVALID (-10001) /**< Empty handle (ctx == NULL) or invalid argument. */
+#define OAKCOMMON_E_STATE (-10002) /**< Call not valid in the current state. */
+#define OAKCOMMON_E_FAILED (-10003) /**< The underlying operation failed. */
+#define OAKCOMMON_E_NOT_FOUND (-10004) /**< Index out of range / entry not found. */
+#define OAKCOMMON_E_NOMEM (-10005) /**< Allocation failed. */
+
+#define SUCCESS OAKCOMMON_OK /**< @deprecated Use OAKCOMMON_OK. */
+
+#endif //OAK_EDITOR_ERROR_H