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/pluginSupport/oliveplugininstance.cpp

@@ -22,7 +22,7 @@
 #include "ofxCore.h"
 #include "ofxMessage.h"
 #include "common/current.h"
-#include "core.h"
+#include "coreengine.h"
 #include "dialog/progress/progress.h"
 #include "node/output/viewer/viewer.h"
 #include "panel/panelmanager.h"
@@ -401,7 +401,7 @@ OfxStatus OlivePluginInstance::editEnd()
 													"Edit Parameters");
 			}
 		}
-		Core::instance()->undo_stack()->push(edit_command_, label);
+		EngineCore::instance()->undo_stack()->push(edit_command_, label);
 		edit_command_ = nullptr;
 		edit_label_.clear();
 		edit_first_label_.clear();
@@ -437,7 +437,7 @@ void OlivePluginInstance::submit_undo_command(UndoCommand *command,
 		return;
 	}
 
-	Core::instance()->undo_stack()->push(command, label);
+	EngineCore::instance()->undo_stack()->push(command, label);
 }
 
 void OlivePluginInstance::progressStart(const std::string &message,