feat(producer): fan out capture across Cloudflare Sandboxes

[tombeckenham]

What

Add a Cloudflare Sandbox adapter that fans heygen-promo-preview-assets capture across up to 10 isolated software-GL Chromes, plus a bench harness that measures wall-clock against the frozen producer baseline.

Why

Local --workers N shares one SwiftShader and was slower on this fixture (+6.4% at 3 workers). Isolated remotes are the path that can actually cut wall-clock. Goal: beat 190,283 ms by 25% (≤ 142,712 ms).

How

Thin wrapper around the existing plan → renderChunk × N → assemble primitives. Custom sandbox image (Chrome headless-shell + FFmpeg + Bun). Fleet pinned to account 86bb57b655af7915f42b29dfc2d8807d, max_instances: 10, instance_type: standard-3. Plan runs remotely so FFmpeg versions match. Not the default local hyperframes render path.

standard-2 (1 vCPU) missed the bar (~239s). standard-3 (2 vCPU) 3-run average 101,587 ms (−46.6%). PSNR 42.7–48.1 dB (bar 30).

Test plan

• Unit tests added/updated (cloudflareSandbox.test.ts — account pin, wrangler cap, resolveChunkPlan(480), mocked plan → renderChunk × N → assemble)
• Manual testing performed (3-run sandbox bench vs golden PSNR)
• Documentation updated (tests/perf/goal.md, tests/perf/README.md)

To reproduce:

cd packages/producer
bun test src/services/distributed/cloudflareSandbox.test.ts
HF_SANDBOX_URL=$HF_SANDBOX_URL bun src/cloudflareSandboxBench.ts --runs 3 \
  --fixture tests/heygen-promo-preview-assets/src