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/render/diskmanager.cpp

@@ -30,7 +30,7 @@
 
 #include "common/filefunctions.h"
 #include "config/config.h"
-#include "core.h"
+#include "coreengine.h"
 #include "dialog/diskcache/diskcachedialog.h"
 
 namespace olive
@@ -370,7 +370,7 @@ bool DiskCacheFolder::delete_least_recent()
 		bool e = delete_file_internal(hash_to_delete);
 
 		if (e) {
-			Core::instance()->warn_cache_full();
+			EngineCore::instance()->warn_cache_full();
 		}
 
 		return e;