perf: overlap independent prototype page QA

[TsekaLuk]

Summary

• separate paid page-image generation from observational Vision QA when locked assignments use distinct provider identities
• keep image concurrency at three, add a bounded QA lane, and join every review before suite completion
• preserve same-provider inline review, cancellation, recovery reuse, exact closure, and Agent-authored scope

Verification

• pnpm test: 1,934 passed, 15 skipped
• pnpm lint
• pnpm build
• pnpm agent:validate
• packaged Rust E2E: 16 passed
• CI-equivalent Playwright: 2 passed
• git diff --check