Front-load post-processing compile into the loader

primeSceneForLoader() compiled scene materials and rendered the shadow/reflection passes
once, but never ran the full composer pipeline — so the SSAO and output passes compiled
their programs and allocated their render targets on the first live frame after the loader
faded, tanking FPS for ~1-2s before it climbed to full.

Now the loader runs composer.render() twice during prime, and precompiles the flip page
materials (created lazily on first flip, so previously missed by renderer.compile) via a
throwaway probe mesh. The heavy first-frame work is paid behind the loader overlay instead.

Verified live: loader timings show composerWarmup taking ~1499ms during load (exactly the
cost that used to hit the first frame); after fade-out there are no over-budget tank frames
in the slow-frame log and idle settles at ~72fps. Static suite passes (165).

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

public/js/webgl-book-lab.js

@@ -4317,12 +4317,39 @@ function primeSceneForLoader() {
     updateTableReflection();
     markLoaderTiming('tableReflection:end');
     markLoaderTiming('shaderCompile:start');
-    renderer.compile(scene, camera);
+    compileFlipMaterialsForLoader();
     markLoaderTiming('shaderCompile:end');
+    // Run the full post-processing pipeline now so the SSAO and output passes compile their
+    // programs and allocate their render targets during the loader, instead of stalling the
+    // first live frames after the loader fades out.
+    markLoaderTiming('composerWarmup:start');
+    if (composer) {
+        composer.render();
+        composer.render();
+    }
+    markLoaderTiming('composerWarmup:end');
     staticSceneBuffersDirty = false;
     markLoaderTiming('primeSceneForLoader:end');
 }
 
+// The flipping page mesh is only created on the first page flip, so its materials are not in
+// the scene graph that renderer.compile walks. Compile them now via a throwaway probe mesh so
+// the first flip does not stutter while the GPU compiles the flip page program.
+function compileFlipMaterialsForLoader() {
+    const probeGeometry = new THREE.PlaneGeometry(0.001, 0.001);
+    const probes = [materials.flipPageSurface, materials.flipPageBackSurface, materials.flipPageEdge]
+        .map((material) => {
+            const probe = new THREE.Mesh(probeGeometry, material);
+            probe.position.copy(book.position);
+            probe.visible = true;
+            book.add(probe);
+            return probe;
+        });
+    renderer.compile(scene, camera);
+    probes.forEach((probe) => book.remove(probe));
+    probeGeometry.dispose();
+}
+
 function tintAmbientFromCanvas(canvas) {
     if (!canvas || !candleBounceLight) return;
     const ctx = canvas.getContext('2d');