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/render/copier.h

@@ -0,0 +1,84 @@
+/***
+
+  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_RENDER_COPIER_H
+#define OAK_EDITOR_RENDER_COPIER_H
+
+#include "node/node.h"
+#include "node/project.h"
+#include "render/error.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Reference-counted handle to a project copier
+ *        (olive::ProjectCopier): deep-copies a project graph for
+ *        background processing (export/precache).
+ *
+ * By-value handle (shared_ptr semantics, see oakcommon's
+ * common/handle.h): oakrender_project_copier_create() returns a handle
+ * with reference count 1; release it with
+ * oakrender_project_copier_free().
+ */
+typedef struct OakRenderProjectCopier {
+	void *ctx; /**< Opaque pointer to the reference-counted object. */
+	void (*addref)(void *ctx); /**< Atomically increments the count. */
+	void (*release)(void *ctx); /**< Decrements the count, destroys at 0. */
+	uint32_t abi_version; /**< OAKRENDER_ABI_VERSION. */
+} OakRenderProjectCopier;
+
+/**
+ * @brief Create a copier. The copy is built by
+ *        oakrender_project_copier_set_project().
+ *
+ * @return Copier handle with reference count 1; ctx is NULL on
+ *         allocation failure.
+ */
+OakRenderProjectCopier oakrender_project_copier_create(void);
+
+/**
+ * @brief Release one reference to a copier; the final release frees the
+ * copier AND its copied project. NULL / empty-handle no-op; clears
+ * copier->ctx after releasing.
+ */
+void oakrender_project_copier_free(OakRenderProjectCopier *copier);
+
+/** @brief (Re)build the copy from `project` (borrowed handle). */
+int oakrender_project_copier_set_project(OakRenderProjectCopier copier,
+										 OakNodeProject project);
+
+/** @brief The copied counterpart of an original node (borrowed handle;
+ *        freeing it only releases the handle box), empty handle when the
+ *        node is not in the copied project. */
+OakNodeNode oakrender_project_copier_get_copy(
+	OakRenderProjectCopier copier, OakNodeNode original);
+
+/** @brief The copied project (borrowed handle; freeing it only releases
+ *        the handle box). */
+OakNodeProject oakrender_project_copier_get_copied_project(
+	OakRenderProjectCopier copier);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif //OAK_EDITOR_RENDER_COPIER_H