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/control.html

@@ -169,6 +169,10 @@
           con la camera lo comandi in “Interattivo Avatar”.</div>
         <label>I miei modelli</label>
         <div id="glb-models" class="glb-models"></div>
+        <label>Apertura braccia <span id="glb-spread-val">0°</span></label>
+        <input type="range" id="glb-spread" min="0" max="35" step="1" value="0" />
+        <div class="hint-mini">Se durante i balli le braccia trapassano il corpo (modelli più larghi
+          del manichino standard), aumenta finché sono libere. Salvato per ogni modello.</div>
         <label>Sfondo</label>
         <select id="glb-bg">
           <option value="gradient">Gradiente</option>