app: PlaybackController + playhead subscription migration (issues 0c, 1-4)

New app/playback/playbackcontroller.{h,cpp}: a lazily created
process-wide singleton whose set_playhead() forwards to
oakengine_viewer_set_playhead and re-broadcasts playhead_changed as a
plain Qt signal. Every app-side oakengine_viewer_set_playhead call site
(17 across viewer, timelinewidget, timelineview, import tool,
timebasedwidget, keyframecontrol, export dialog, mainwindow) now goes
through it.

Migrate the first four playhead subscribers off the raw C event
subscription / EngineEventBridge to the controller signal:
- TimeBasedView (issue 1)
- NodeParamViewWidgetBridge (issue 2)
- NodeParamViewKeyframeControl (issue 3)
- NodeParamViewConnectedLabel (issue 4)

Each stores a QMetaObject::Connection filtered on its viewer node and
disconnects on teardown; the viewer_sub_ members are gone. Plan docs
mark issues 0c and 1-4 as done.

Closes #30, closes #31, closes #32, closes #51, closes #52.

app/widget/nodeparamview/nodeparamviewconnectedlabel.cpp

@@ -21,6 +21,8 @@
 
 #include "nodeparamviewconnectedlabel.h"
 
+#include "playback/playbackcontroller.h"
+
 #include <QHBoxLayout>
 
 #include "oakutil/qtutils.h"
@@ -112,30 +114,26 @@ NodeParamViewConnectedLabel::NodeParamViewConnectedLabel(const oak::Input &input
 
 NodeParamViewConnectedLabel::~NodeParamViewConnectedLabel()
 {
-	// Raw C-API subscription carries `this` as userdata; not covered by
-	// Qt's auto-disconnect. Unsubscribe or the next playhead event calls
-	// into a dead object.
+	// Drop the PlaybackController connection (Qt would auto-disconnect
+	// anyway, but keep the symmetric teardown).
 	set_viewer_node(nullptr);
 }
 
 void NodeParamViewConnectedLabel::set_viewer_node(OakEngineNode *viewer)
 {
-	if (viewer_) {
-		oakengine_event_unsubscribe(viewer_sub_);
-		viewer_sub_ = 0;
-	}
+	disconnect(viewer_conn_);
 
 	viewer_ = viewer;
 
 	if (viewer_) {
-		viewer_sub_ = oakengine_event_subscribe(
-			viewer_,
-			OAKENGINE_EVENT_VIEWER_PLAYHEAD_CHANGED,
-			[](const oakengine_event *, void *userdata) {
-				static_cast<NodeParamViewConnectedLabel *>(userdata)
-					->update_value_tree();
-			},
-			this);
+		viewer_conn_ = connect(
+			PlaybackController::instance(),
+			&PlaybackController::playhead_changed, this,
+			[this](OakEngineNode *n, const core::Rational &) {
+				if (n == viewer_) {
+					update_value_tree();
+				}
+			});
 		update_value_tree();
 	}
 }