Per-model arm spread against limb/body intersection

Dance clips are authored for the standard mannequin: on chunkier models
the same joint angles drive the arms through the torso (the ninja is
clean because its proportions match). New 'Apertura braccia' slider
(0-35deg, saved per model path): the upper arms get an extra outward
rotation about the world Z axis during clip playback (left +, right -),
solved back into each bone's local frame through the parent rotation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/output.js

@@ -323,6 +323,7 @@ djv.onControl(async (m) => {
     case 'modelBg': viz.modelBg = m.mode || 'gradient'; break;
     case 'glbAnims': viz.setClipFilter(m.names || []); break;
     case 'modelBpm': viz.setManualBpm(m.bpm || 0); break;
+    case 'modelSpread': viz.setArmSpread(m.deg || 0); break;
     case 'animAdd':
       // extra animation GLB for the dance library
       try {