build: split the engine into liboakengine.so; worker drops the UI entirely

Physical split: app/{audio,cli,codec,common,config,node,pluginSupport,
render,task,timeline,undo,tool,shaders} plus coreengine, version and
ui/icons+colorcoding move to a new top-level engine/ tree, built as
liboakengine.so (shared). The render backends (oakgl/oakvulkan) move
with it and link the engine library instead of embedding a static
render-core subset (libolive-rendercore is gone).

- oak-render-worker now links liboakengine instead of the whole
  libolive-editor object set: 336MB -> 2.9MB, no Qt Widgets UI
- the editor links liboakengine for the engine and keeps only UI
  objects in libolive-editor
- install/packaging: GNUInstallDirs libdir on Linux, bundle copy on
  macOS, oakengine.dll staged for NSIS, AppImage validation entry
- fix backend lookup for the new layout: DynamicRenderer searched
  ../app but backends now live in engine/; a stale pre-split liboakgl
  in the build tree got dlopened instead, re-initialized and later
  destroyed the interposed engine statics (full-suite segfault at
  DialogSequenceParameterTab, found via gdb watchpoint)

engine/task/proxy/proxy.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_PROXYTASK_H
+#define OAK_PROXYTASK_H
+
+#include "codec/proxymanager.h"
+#include "task/task.h"
+
+namespace olive
+{
+
+class ProxyTask : public Task {
+	Q_OBJECT
+public:
+	ProxyTask(const QString &source_filename, int stream_index,
+			  const ProxyManager::ProxyParams &params,
+			  const QString &output_filename);
+
+	/**
+	 * @brief Builds the ffmpeg command line for a proxy generation run
+	 *
+	 * Extracted for testability. The video stream is always mapped first so
+	 * that it is stream 0 in the proxy file; audio streams (when enabled)
+	 * follow in source order.
+	 */
+	static QStringList build_arguments(const QString &source_filename,
+									  int stream_index,
+									  const ProxyManager::ProxyParams &params,
+									  const QString &output_filename);
+
+	/**
+	 * @brief Parses one line of ffmpeg "-progress" output
+	 *
+	 * Extracted for testability. If the line carries an output timestamp
+	 * ("out_time_us=" or "out_time_ms="), returns the progress fraction in
+	 * the range [0, 1] against duration_seconds. Returns a negative value
+	 * when the line carries no timestamp or duration_seconds is unknown.
+	 */
+	static double parse_progress(const QString &line, double duration_seconds);
+
+protected:
+	virtual bool run() override;
+
+private:
+	QString source_filename_;
+	int stream_index_;
+	ProxyManager::ProxyParams params_;
+	QString output_filename_;
+};
+
+}
+
+#endif // OAK_PROXYTASK_H