fix(shader-transitions): page-side render dropped shader transitions and final-scene content

Page-side compositing (default on) silently dropped HyperShader.init shader
transitions in the engine render. The compositor clones the from/to scenes to
feed drawElementImage, but cloneNode copies the GSAP opacity-fade, and Chrome
won't paint hidden elements, so drawElementImage throws "No cached paint record"
and the shader degrades to a hard cut. Force the clones visible before capture,
as the html2canvas path already does via forceSceneVisibleInClone.

Also fixes the final scene's content dropping in the last beat: the core clip
runtime hides it shortly before the composition ends, and page-side screenshots
the live page (the layered path survives via forceVisible per-scene capture).
Un-hide the settled scene on non-transition frames.

Page-side only; retains the ~6.5x page-side speedup.

packages/shader-transitions/src/engineModePageComposite.ts

@@ -196,6 +196,16 @@ export function installPageSideCompositor(options: PageCompositorInstallOptions)
     return null;
   }
 
+  // Scene on screen at a non-transition time: after the last transition whose
+  // window has passed. Full transitions list so the index matches scene order.
+  function settledSceneIdAt(time: number): string | undefined {
+    let idx = 0;
+    for (const t of transitions) {
+      if (time >= t.time + (t.duration ?? defaultDuration)) idx += 1;
+    }
+    return scenes[Math.min(idx, scenes.length - 1)];
+  }
+
   let currentActive: ResolvedTransition | null = null;
   let currentProgress = 0;
 
@@ -224,8 +234,24 @@ export function installPageSideCompositor(options: PageCompositorInstallOptions)
     }
     while (fromStaging.firstChild) fromStaging.removeChild(fromStaging.firstChild);
     while (toStaging.firstChild) toStaging.removeChild(toStaging.firstChild);
-    fromStaging.appendChild(fromEl.cloneNode(true));
-    toStaging.appendChild(toEl.cloneNode(true));
+    const fromClone = fromEl.cloneNode(true) as HTMLElement;
+    const toClone = toEl.cloneNode(true) as HTMLElement;
+    fromStaging.appendChild(fromClone);
+    toStaging.appendChild(toClone);
+
+    // cloneNode copies the GSAP opacity-fade (opacity:0 / hidden data-start), and
+    // Chrome won't paint hidden elements — drawElementImage then throws "No cached
+    // paint record" and the shader degrades to a hard cut. The shader blends from
+    // full-opacity textures via u_progress, so force the clones visible. Cf.
+    // forceSceneVisibleInClone (html2canvas path).
+    for (const clone of [fromClone, toClone]) {
+      clone.style.opacity = "1";
+      clone.style.visibility = "visible";
+      clone.querySelectorAll<HTMLElement>("[data-start]").forEach((el) => {
+        el.style.opacity = "1";
+        el.style.visibility = "visible";
+      });
+    }
 
     // Decode any data-URI images in clones so the browser has current
     // bitmaps before the micro-screenshot forces a paint pass.
@@ -326,6 +352,14 @@ export function installPageSideCompositor(options: PageCompositorInstallOptions)
         pWin.__hf_page_composite_pending = false;
         while (fromStaging.firstChild) fromStaging.removeChild(fromStaging.firstChild);
         while (toStaging.firstChild) toStaging.removeChild(toStaging.firstChild);
+        // Live-page screenshot parity with the layered path's forceVisible: the
+        // core clip runtime hides the final scene a beat before the comp ends, so
+        // un-hide the settled scene (others stay at opacity 0).
+        const settledId = settledSceneIdAt(time);
+        const settled = settledId ? document.getElementById(settledId) : null;
+        if (settled instanceof HTMLElement && settled.style.visibility === "hidden") {
+          settled.style.visibility = "visible";
+        }
         return result;
       }
       currentActive = active;