LED wall active area: confine the output to a pixel-mapping rectangle

New 'Area LED' controls in the Schermo tab: W x H @ X,Y. Everything
(canvas, videos, images, logos, ticker) lives in a #stage box whose
transform makes it the containing block for the fixed layers, so resizing
it confines the whole show at once, black outside. The canvas resizes to
the stage, so shaders render at the wall's native resolution. Persisted
in localStorage and re-pushed when the output window (re)starts; the
recorder follows the canvas position.

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

src/control.html

@@ -344,6 +344,18 @@
           <button id="btn-fullscreen">⛶ Fullscreen</button>
         </div>
       </section>
+      <section class="span2">
+        <label>Area LED (pixel mapping)</label>
+        <div class="row led-row">
+          <label class="led-chk"><input type="checkbox" id="led-on" /> Attiva</label>
+          <input type="number" class="led-num" id="led-w" min="16" max="8192" step="1" title="Larghezza in px" /> ×
+          <input type="number" class="led-num" id="led-h" min="16" max="8192" step="1" title="Altezza in px" /> px @
+          <input type="number" class="led-num" id="led-x" min="0" max="8192" step="1" title="Posizione X in px" /> ,
+          <input type="number" class="led-num" id="led-y" min="0" max="8192" step="1" title="Posizione Y in px" />
+        </div>
+        <div class="hint-mini">Per processori LED con mapping 1:1: confina tutto l'output in un rettangolo
+          Larghezza×Altezza alla posizione X,Y sullo schermo (nero attorno). Es. wall 896×512 → 896 × 512 @ 0 , 0.</div>
+      </section>
     </div>
 
   </main>