Expose the yard's hemisphere light for skyfx

Lane C's skyfx modulates sun and hemi as the storm builds and hands them back on
dispose() — it doesn't own them. It already had `sun`; `hemi` was private, so the
sky could darken but the sky-bounce fill couldn't follow it down.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/world/js/world.js

@@ -343,7 +343,11 @@ export function createWorld(scene, opts = {}) {
     sunDir: SUN_DIR.clone(),
     solids,
     root,
+    // Lane C's skyfx MODULATES these as the storm builds and hands them back
+    // untouched on dispose() — it doesn't own them. That's why the yard exposes
+    // its lights rather than keeping them private.
     sun,
+    hemi,
 
     /** @param {string} id */
     anchor(id) {