fix: make the reading viewport a script-driven scroll container so sticky and view timelines work

src/styles/world.css

@@ -524,12 +524,12 @@
 .reading-stage[data-reading-stage='live'] > .reading-viewport {
   position: sticky;
   top: 0;
-  overflow: clip;
+  /* A real scroll container (scrolled from script, never by the user), so
+     sticky children and view timelines resolve against the reading window. */
+  overflow: hidden;
+  overscroll-behavior: none;
   isolation: isolate;
 }
-.reading-stage[data-reading-stage='live'] .portfolio-main {
-  will-change: transform;
-}
 
 /* The stationary reading edge supplies the live reveal. View timelines use
    layout positions, which no longer follow the translated content here. */