fix(sound): stop play-scene via doStopAll

Final whole-branch review caught a cross-task defect: ~doScenePlay
started ~bpEngine but registered no cleanup, so ~doStopAll (run by
~doScene at every scene switch, and by \stop) never stopped the
engine -- body-play kept playing over any other scene, breaking the
"\stop coupe tout" contract. Register the stop via ~doRegisterPriv
(name, killFn) like every other scene. Verified on macm1: doScene(\play)
starts the engine, doScene(\stop) stops it.

sound_algo/data_only/scene_pose_play.scd

@@ -163,7 +163,7 @@
 );
 
 // -- 7) Enregistrement scene -------------------------------------------
-~doScenePlay = { ~bpEngine[\start].() };
+~doScenePlay = { ~bpEngine[\start].(); ~doRegisterPriv.(\play, { ~bpEngine[\stop].() }) };
 
 "[data-only/scene_pose_play] modes + scale ready".postln;
 )