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/node/color/colormanager/colormanager.cpp

@@ -27,7 +27,7 @@
 #include "common/define.h"
 #include "common/filefunctions.h"
 #include "config/config.h"
-#include "core.h"
+#include "node/project.h"
 
 namespace olive
 {

app/node/color/ociolut/ociolut.cpp

@@ -25,7 +25,7 @@
 #include <QMetaObject>
 
 
-#include "core.h"
+#include "coreengine.h"
 #include "node/color/colormanager/colormanager.h"
 #include "render/lutlibrary.h"
 #include "render/previewautocacher.h"
@@ -204,8 +204,8 @@ void OCIOLutNode::set_last_error(const QString &error) const
 
 	// Make the error visible to the user instead of failing silently, but only
 	// from the main process (the render worker has no status bar)
-	if (!error.isEmpty() && is_main_process() && Core::instance()) {
-		Core::instance()->show_status_bar_message(error, 10000);
+	if (!error.isEmpty() && is_main_process() && EngineCore::instance()) {
+		EngineCore::instance()->show_status_bar_message(error, 10000);
 	}
 }
 

app/node/distort/cornerpin/cornerpindistortnode.cpp

@@ -22,7 +22,6 @@
 #include "cornerpindistortnode.h"
 
 #include "common/lerp.h"
-#include "core.h"
 
 namespace olive
 {

app/node/distort/crop/cropdistortnode.cpp

@@ -22,7 +22,6 @@
 #include "cropdistortnode.h"
 
 #include "common/util.h"
-#include "core.h"
 #include "node/sliderdisplaytype.h"
 
 namespace olive

app/node/generator/shape/shapenodebase.cpp

@@ -25,7 +25,6 @@
 #include <QVector2D>
 
 #include "common/util.h"
-#include "core.h"
 #include "node/nodeundo.h"
 
 namespace olive

app/node/generator/text/textv3.cpp

@@ -26,7 +26,7 @@
 #include <QTextDocument>
 
 #include "common/html.h"
-#include "core.h"
+#include "coreengine.h"
 #include "node/project.h"
 #include "node/nodeundo.h"
 
@@ -300,7 +300,7 @@ void TextGeneratorV3::gizmo_deactivated()
 
 void TextGeneratorV3::set_vertical_alignment_undoable(Qt::Alignment a)
 {
-	Core::instance()->undo_stack()->push(
+	EngineCore::instance()->undo_stack()->push(
 		new NodeParamSetStandardValueCommand(NodeInput(this,
 													   k_vertical_alignment_input),
 											 get_our_alignment_from_qts(a)),

app/node/gizmo/text.cpp

@@ -21,7 +21,7 @@
 
 #include "text.h"
 
-#include "core.h"
+#include "coreengine.h"
 #include "undo/undocommand.h"
 
 namespace olive
@@ -45,7 +45,7 @@ void TextGizmo::update_input_html(const QString &s, const Rational &time)
 		MultiUndoCommand *command = new MultiUndoCommand();
 		Node::set_value_at_time(input_.input(), time, s, input_.track(), command,
 							 true);
-		Core::instance()->undo_stack()->push(command, tr("Edit Text"));
+		EngineCore::instance()->undo_stack()->push(command, tr("Edit Text"));
 	}
 }
 

app/node/inputdragger.cpp

@@ -21,7 +21,6 @@
 
 #include "inputdragger.h"
 
-#include "core.h"
 #include "node.h"
 #include "nodeundo.h"
 

app/node/node.cpp

@@ -27,7 +27,6 @@
 #include <QFile>
 
 #include "common/lerp.h"
-#include "core.h"
 #include "config/config.h"
 #include "node/group/group.h"
 #include "node/project/serializer/typeserializer.h"

app/node/output/viewer/viewer.cpp

@@ -22,7 +22,7 @@
 #include "viewer.h"
 
 #include "config/config.h"
-#include "core.h"
+#include "coreengine.h"
 #include "node/traverser.h"
 
 namespace olive
@@ -107,7 +107,7 @@ QVariant ViewerOutput::data(const DataType &d) const
 	case duration: {
 		Rational using_timebase;
 		Timecode::Display using_display =
-			Core::instance()->get_timecode_display();
+			EngineCore::instance()->get_timecode_display();
 
 		// Get first enabled streams
 		VideoParams video = get_first_enabled_video_stream();

app/node/project.cpp

@@ -27,7 +27,6 @@
 #include "common/current.h"
 #include "common/qtutils.h"
 #include "common/xmlutils.h"
-#include "core.h"
 #include "node/color/ociobase/ociobase.h"
 #include "node/factory.h"
 #include "node/group/group.h"

app/node/project/footage/footage.cpp

@@ -27,13 +27,15 @@
 #include <QImage>
 #include <QPainter>
 #include <QStandardPaths>
+#include <QTimer>
 
 #include "codec/decoder.h"
 #include "common/filefunctions.h"
 #include "common/qtutils.h"
 #include "common/xmlutils.h"
 #include "config/config.h"
-#include "core.h"
+#include "node/color/colormanager/colormanager.h"
+#include "node/project.h"
 #include "render/job/footagejob.h"
 #include "ui/icons/icons.h"
 

app/node/project/serializer/serializer.cpp

@@ -26,7 +26,7 @@
 #include <QXmlStreamReader>
 
 #include "common/xmlutils.h"
-#include "core.h"
+#include "coreengine.h"
 #include "node/group/group.h"
 #include "serializer190219.h"
 #include "serializer210528.h"
@@ -156,7 +156,7 @@ ProjectSerializer::Result ProjectSerializer::load(Project *project,
 ProjectSerializer::Result ProjectSerializer::paste(LoadType load_type,
 												   Project *project)
 {
-	QString clipboard = Core::paste_string_from_clipboard();
+	QString clipboard = EngineCore::paste_string_from_clipboard();
 	if (clipboard.isEmpty()) {
 		return k_no_data;
 	}
@@ -257,7 +257,7 @@ ProjectSerializer::Result ProjectSerializer::copy(const SaveData &data)
 	ProjectSerializer::Result res = ProjectSerializer::save(&writer, data);
 
 	if (res == k_success) {
-		Core::copy_string_to_clipboard(copy_str);
+		EngineCore::copy_string_to_clipboard(copy_str);
 	}
 
 	return res;