fix(engine): warm-up seek after session init to prevent parallel worker flash frames

[miguel-heygen]

Problem

When rendering with parallel workers (-w 5 or auto), compositions using async GLTF model loading + Three.js MeshPhysicalMaterial with transmission: 1 produce a single flash frame at worker boundaries. The flash shows the wrong 3D object orientation (e.g., iPhone front when it should show back) for exactly one frame.

Root cause: Each parallel worker loads the page in a separate Chrome tab. After the page is ready (window.__hf.duration > 0), the first frame captured by a worker uses an uninitialized WebGL transmission render target. Three.js's transmission pipeline (MeshPhysicalMaterial.transmission) requires at least one prior render to populate its internal render targets — without it, the first render produces incorrect output.

Reproduction: Render any composition with async-loaded GLTF models + glass/transmissive materials using 2+ workers. The flash appears at worker boundary frames (e.g., frame 90, 180, 270, 360 with 5 workers at 30fps).

Works with 1 worker because all frames are captured sequentially from a single page — the first frame at t=0 primes the pipeline, and subsequent frames reuse the initialized state.

Fix

After session initialization in frameCapture.ts (initializeSession), perform three warm-up seeks via the __hf protocol:

await page.evaluate(`window.__hf?.seek?.(0)`);
await page.evaluate(`window.__hf?.seek?.(window.__hf?.duration * 0.5 || 0)`);
await page.evaluate(`window.__hf?.seek?.(0)`);

This primes the page's render pipeline at t=0, t=midpoint, and t=0 again — ensuring WebGL buffers, transmission render targets, and any lazy-initialized GPU resources are fully populated before real frame capture begins.

Applied to both capture modes (screenshot and beginFrame).

Changes

File	Change
packages/engine/src/services/frameCapture.ts	Add 3 warm-up seeks after session init in both screenshot and beginFrame paths (+16 lines)

Test plan

• All 513 engine tests pass (2 pre-existing failures from missing generated file, unrelated)
• Render GLTF composition with -w 5 — verify no flash frames at worker boundaries
• Render standard (non-GLTF) composition with -w 5 — verify no regression
• Render with -w 1 — verify still works (warm-up is harmless for single worker)