radiance: pointer/touch-reactive particles

Stable-fluids solver (velocity + dye) stirred by mouse/touch drives a GPU
spring system over all ~65k particles: they swirl with the flow, heat up
and flare while disturbed, then spring home. The sky shader leans and
washes with the flow. WebGL2; falls back to the static orb without float
render targets, and to a CSS orb without WebGL2.

src/styles/radiance.css

@@ -27,6 +27,7 @@ img { display: block; }
 
 /* ---------- hero ---------- */
 .hero {
+  touch-action: none;
   position: fixed; inset: 0;
   background:
     radial-gradient(115% 80% at 50% 40%, transparent 45%, rgb(251 188 166 / 0.65) 100%),