Interactive camera family: webcam motion pushes physical balls on screen

New 'Interattivo Palle' family (interactive.js): frame-differencing motion map
from the webcam, ball physics (push-away forces, collisions, beat kicks),
palette-tinted rendering with motion mist. Camera is requested lazily and
released when switching preset; mouse/touch fallback when unavailable.
Excluded from Auto VJ so the camera never turns on by itself.

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

src/output.html

@@ -43,6 +43,7 @@
   <script src="shaders.js"></script>
   <script src="effects.js"></script>
   <script src="fluid.js"></script>
+  <script src="interactive.js"></script>
   <script src="visualizer.js"></script>
   <script src="audio.js"></script>
   <script src="output.js"></script>