refactor(producer): apply /simplify findings on variables-prod

Align the no-op timeline duration with the root's data-duration. The
fixture's root has data-duration="3" but the placeholder timeline tween
was still { duration: 2 } — leftover from when I bumped the duration
from 2s to 3s to dodge the PSNR-checkpoint-at-1.99s parse edge case.
The tween is a no-op (no targets, no visible effect) so rendered pixels
don't change; baseline still passes Docker regression at 100/100
checkpoints.

Reuse + efficiency reviews otherwise clean. Two findings deferred:
silence.wav duplication is real but only 2 fixtures share it today —
worth extracting to tests/_shared/ when the third fixture lands.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/producer/tests/variables-prod/src/index.html

@@ -89,7 +89,7 @@
       const tl = gsap.timeline({ paused: true });
       // Static placement — no animation. Keeps the regression frame-stable
       // and isolates "did the variables flow through?" from motion concerns.
-      tl.to({}, { duration: 2 });
+      tl.to({}, { duration: 3 });
       window.__timelines["main"] = tl;
     </script>
   </body>