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/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)),