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/plugin/pluginrenderer.cpp

@@ -41,7 +41,7 @@
 #include <QVector3D>
 #include <QVector4D>
 #include "pluginrenderer.h"
-#include "core.h"
+#include "coreengine.h"
 #include "undo/undostack.h"
 #include "pluginSupport/oliveclip.h"
 #include "pluginSupport/oliveplugininstance.h"
@@ -1350,7 +1350,7 @@ static void mark_render_failure(olive::TexturePtr destination)
 /// Show an error dialog and undo the last operation. Must be called from the GUI thread.
 static void show_error_dialog_and_undo(const QString &message)
 {
-	if (auto *core = olive::Core::instance()) {
+	if (auto *core = olive::EngineCore::instance()) {
 		if (auto *stack = core->undo_stack()) {
 			if (stack->can_undo()) {
 				stack->undo();