refactor: split Core into EngineCore (engine) and Core (UI)

EngineCore (new app/coreengine.{h,cpp}) owns every engine-safe part of
the old Core singleton: CoreParams, lifecycle of the engine managers,
UndoStack, tool/snapping/timecode state, locale, autorecovery, recent
projects, footage filters, clipboard, project registry, type
declarations, and the proxy toggle. UI dependencies are inverted
through hooks instead: status-bar/cache-full signals and std::function
handlers for image-sequence confirmation, footage relink, OTIO import,
project save/close and layout load (same pattern as
Config::ErrorHandler).

Core (app/) now derives from EngineCore and keeps only UI behavior:
the main window, dialogs, panel heuristics, import/export flows and
project lifecycle presentation. Its public API is unchanged (all
inherited), and Core::instance() covariantly static_casts the engine
singleton. The render worker constructs EngineCore directly, making it
the first binary that no longer needs the UI side of Core.

~25 engine call sites move from core.h to coreengine.h; a dozen more
drop a vestigial core.h include (gaining direct includes for symbols
they were borrowing transitively). Full gtest suite green (1986 tests,
0 failures).

app/task/project/import/import.cpp

@@ -25,7 +25,7 @@
 #include <QFileInfo>
 
 #include "config/config.h"
-#include "core.h"
+#include "coreengine.h"
 #include "node/nodeundo.h"
 #include "node/project/footage/footage.h"
 
@@ -41,7 +41,7 @@ ProjectImportTask::ProjectImportTask(Folder *folder,
 		filenames_.append(QFileInfo(f));
 	}
 
-	file_count_ = Core::count_files_in_file_list(filenames_);
+	file_count_ = EngineCore::count_files_in_file_list(filenames_);
 
 	set_title(tr("Importing %n file(s)", nullptr, file_count_));
 }
@@ -176,7 +176,7 @@ void ProjectImportTask::validate_image_sequence(Footage *footage,
 			// user just in case...
 			bool is_sequence;
 
-			QMetaObject::invokeMethod(Core::instance(), "confirm_image_sequence",
+			QMetaObject::invokeMethod(EngineCore::instance(), "confirm_image_sequence",
 									  Qt::BlockingQueuedConnection,
 									  Q_RETURN_ARG(bool, is_sequence),
 									  Q_ARG(QString, footage->filename()));