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/plugin/host.h

@@ -0,0 +1,69 @@
+/***
+
+  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_PLUGIN_HOST_H
+#define OAK_EDITOR_PLUGIN_HOST_H
+
+#include "plugin/error.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Initialize the OFX host (olive::plugin::load_plugins() with the
+ *        default search paths). Idempotent.
+ */
+int oakplugin_host_init(void);
+
+/** @brief Shut the host down (persistent messages cleared). */
+void oakplugin_host_shutdown(void);
+
+/** @brief Scan additional bundle directories. */
+int oakplugin_host_scan(const char *const *bundle_dirs, int dir_count);
+
+/** @brief Number of discovered plugins (>= 0), or a negative error. */
+int oakplugin_host_plugin_count(void);
+
+/** @brief Plugin identifier at index (two-stage string getter). */
+int oakplugin_host_plugin_id_at(int index, char *buf, int buf_size);
+
+/** @brief Plugin label for an identifier (two-stage; currently the
+ *        identifier itself). OAKPLUGIN_E_NOT_FOUND for unknown ids. */
+int oakplugin_host_plugin_label(const char *plugin_id, char *buf,
+								int buf_size);
+
+/**
+ * @brief UI message handler for OFX host messages (question replies use
+ *        OAKPLUGIN_MESSAGE_ANSWER_YES/NO). Without a handler, messages
+ *        are logged and questions get "no".
+ */
+#define OAKPLUGIN_MESSAGE_ANSWER_NO 0
+#define OAKPLUGIN_MESSAGE_ANSWER_YES 1
+typedef int (*oakplugin_message_fn)(const char *type, const char *message,
+									void *userdata);
+void oakplugin_host_set_message_handler(oakplugin_message_fn fn,
+										void *userdata);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif //OAK_EDITOR_PLUGIN_HOST_H