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.

docs/zh/plans/eliminate-event-bridge-issues.md

@@ -67,7 +67,7 @@ events; C callbacks just queue to the GUI thread and re-emit typed Qt signals.
   must use it instead of their own subscriptions.
 - **必做 / Required：build + ctest 全绿。**
 
-### issue 0c — 建 app 内 PlaybackController（1 天）
+### issue 0c ✅ (done) — 建 app 内 PlaybackController（1 天）
 新建 `app/playback/playbackcontroller.{h,cpp}`，信号
 `playhead_changed(oak::Node viewer, Rational)`。
 把 app 内所有 `oakengine_viewer_set_playhead` 调用点（ViewerWidget 播放循环、
@@ -83,23 +83,23 @@ goes through; it re-broadcasts `playhead_changed` as an app-internal Qt signal.
 
 ## playhead 族迁移 / Playhead migrations（模式 A，约半天/个）
 
-### issue 1 — timebasedview 的 playhead 订阅
+### issue 1 ✅ (done) — timebasedview 的 playhead 订阅
 `app/widget/timebased/timebasedview.cpp:168`（裸 C 回调）。改连
 PlaybackController::playhead_changed。
 - 验收 / Acceptance：播放时 ruler 播放头线正常移动；无裸 C 订阅残留。
 - **必做 / Required：build + ctest 全绿。**
 
-### issue 2 — NodeParamViewWidgetBridge playhead
+### issue 2 ✅ (done) — NodeParamViewWidgetBridge playhead
 `app/widget/nodeparamview/nodeparamviewwidgetbridge.cpp:1140`（裸 C）。
 - 验收 / Acceptance：播放时关键帧插值滑条值随播放头更新。
 - **必做 / Required：build + ctest 全绿。**
 
-### issue 3 — NodeParamViewKeyframeControl playhead
+### issue 3 ✅ (done) — NodeParamViewKeyframeControl playhead
 `app/widget/nodeparamview/nodeparamviewkeyframecontrol.cpp:222`（裸 C）。
 - 验收 / Acceptance：播放头移动时 prev/next/toggle 关键帧按钮状态正确。
 - **必做 / Required：build + ctest 全绿。**
 
-### issue 4 — NodeParamViewConnectedLabel playhead
+### issue 4 ✅ (done) — NodeParamViewConnectedLabel playhead
 `app/widget/nodeparamview/nodeparamviewconnectedlabel.cpp:131`（裸 C）。
 - 验收 / Acceptance：值树随播放头刷新。
 - **必做 / Required：build + ctest 全绿。**