fix(shader-transitions,producer): harden CSS-only transition lifecycle and unblock CI

Three follow-on fixes after the optional-shader change rebased onto current
main (PR #832 introduced page-side compositing and the producer's hf#732
layered pipeline since this PR was opened).

shader-transitions/hyper-shader.ts
- Treat `cache.prog === null` as the canonical immutable marker for
  CSS-only transitions via a new `isCssOnlyTransition()` helper.
- `disposeCachedTransition()` now restores the always-ready CSS fallback
  state for prog=null caches instead of zeroing `fallback`/`ready` — the
  previous behaviour, combined with `markScenesDirty()` re-running the
  prewarm/capture pipeline, could put a CSS-only cache through the WebGL
  path and reach `renderShader(state.prog!)` with a null prog (Copilot
  review on lines 1168 + 1319).
- `markScenesDirty()` skips CSS-only caches; they have no shader to
  recompile and no texture pyramid to recapture.
- `ensureTransitionCachesReady()` filters CSS-only caches out of the
  prewarm work list.
- `tickShader()` now routes on `cache.fallback || cache.prog === null`
  and threads a narrowed non-null `prog` local into `renderShader()`,
  removing the unsound `state.prog!` non-null assertion.
- `initEngineMode()` filters CSS-only transitions before passing them to
  `installPageSideCompositor()`, which expects `shader: ShaderName`
  (required). Page-side compositing is shader-only; CSS crossfades stay
  on the GSAP opacity timeline.

producer/render/stages/captureHdrHybridLoop.ts
producer/render/stages/captureHdrSequentialLoop.ts
- Guard `activeTransition.shader` against undefined: when omitted, route
  the Node-side blend through `crossfade` (the engine's canonical
  opacity blend, equivalent to `applyFallbackTransition()` on the page).
- The hybrid path also bypasses the worker pool when `shaderName` is
  absent and runs `crossfade` inline.

This addresses the Copilot review comments and unblocks the 5 failing CI
jobs (Build, Typecheck, CLI smoke, Windows tests, Windows render) which
all rooted in 4 TS errors at these exact sites.

Co-authored-by: Cursor <cursoragent@cursor.com>

packages/producer/src/services/render/stages/captureHdrHybridLoop.ts

@@ -260,11 +260,17 @@ export async function runHybridLayeredFrameLoop(input: HybridLoopInput): Promise
           // awaits it. The encoder reorder buffer fences ordering so out-
           // of-order blend completion is fine.
           const frameIdx = i;
+          // When the @hyperframes/shader-transitions composition omits the
+          // shader on a transition entry, it requests a CSS crossfade. The
+          // engine-side path uses applyFallbackTransition() on the page; the
+          // producer's Node-side layered pipeline runs the equivalent here
+          // by routing the blend through `crossfade`.
+          const shaderName = activeTransition.shader;
           const dispatch: Promise<void> = (async () => {
-            if (poolRef) {
+            if (poolRef && shaderName) {
               const blendStart = Date.now();
               const result = await poolRef.run({
-                shader: activeTransition.shader,
+                shader: shaderName,
                 bufferA: buffers.bufferA,
                 bufferB: buffers.bufferB,
                 output: buffers.output,
@@ -277,7 +283,9 @@ export async function runHybridLayeredFrameLoop(input: HybridLoopInput): Promise
               buffers.output = result.output;
               addHdrTiming(hdrPerf, "transitionCompositeMs", blendStart);
             } else {
-              const transitionFn: TransitionFn = TRANSITIONS[activeTransition.shader] ?? crossfade;
+              const transitionFn: TransitionFn = shaderName
+                ? (TRANSITIONS[shaderName] ?? crossfade)
+                : crossfade;
               const blendStart = Date.now();
               transitionFn(
                 buffers.bufferA,

packages/producer/src/services/render/stages/captureHdrSequentialLoop.ts

@@ -172,7 +172,13 @@ export async function runSequentialLayeredFrameLoop(input: SequentialLoopInput):
         });
       }
 
-      const transitionFn: TransitionFn = TRANSITIONS[activeTransition.shader] ?? crossfade;
+      // CSS-crossfade transitions (shader omitted in the composition) take
+      // the same Node-side blend path — `crossfade` is the engine's
+      // canonical opacity blend, equivalent to applyFallbackTransition().
+      const shaderName = activeTransition.shader;
+      const transitionFn: TransitionFn = shaderName
+        ? (TRANSITIONS[shaderName] ?? crossfade)
+        : crossfade;
       transitionFn(
         transitionBuffers.bufferA,
         transitionBuffers.bufferB,