test(producer): guard removed autoAlpha fast-capture shim

[miguel-heygen]

Summary

• Adds regression coverage that the shipped HF early stub no longer contains the obsolete __HF_FAST_CAPTURE_AUTOALPHA__ / HF_FAST_CAPTURE_AUTOALPHA sentinel.
• Pins both former rewrite entry points: top-level gsap.set(...) and batched timeline calls keep authored { opacity: ... } vars even if a page defines the obsolete flag.

Investigation

• Current main already has no runtime __HF_FAST_CAPTURE_AUTOALPHA__ shim and no engine-side page.evaluateOnNewDocument setter for that flag. git grep is clean for the flag.
• Historical commits 13b8b0e5 / 0ef2643b added the rewrite in packages/producer/stubs/hf-early-stub.ts; current early stub is back to timeline batching only.
• Chromium issue page is auth-gated from this environment, so I validated behavior directly. Raw drawElementImage repro with stacked opacity-animated absolute layers produced expected pixels and zero black pixels on cached headless-shell 146.0.7680.153, 148.0.7778.97, and 150.0.7871.24.

Verification

• bun test packages/producer/src/services/fileServer.test.ts
• bun --filter @hyperframes/producer typecheck
• bunx oxfmt --check packages/producer/src/services/fileServer.test.ts
• git grep -n "__HF_FAST_CAPTURE_AUTOALPHA__\|HF_FAST_CAPTURE_AUTOALPHA" HEAD -- . (no matches)