Phase 2: Stream responses to client via StreamingBody

[aram356]

Parent: #563
Depends on: #566 (Phase 1)

Goal

Wire up Fastly's StreamingBody API so processed chunks flow directly to the client instead of being buffered.

Tasks

• Phase 2, Task 6: Migrate entry point from #[fastly::main] to raw main() #573 — Task 6: Migrate entry point from #[fastly::main] to raw main()
• Phase 2, Task 7: Refactor process_response_streaming to accept W: Write #574 — Task 7: Refactor process_response_streaming to accept W: Write
• Phase 2, Task 8: Add streaming path to publisher proxy #575 — Task 8: Add streaming path to publisher proxy
• Phase 2, Task 9: Full verification #576 — Task 9: Phase 2 full verification
• Phase 2, Task 10: Chrome DevTools MCP baseline and comparison #577 — Task 10: Chrome DevTools MCP baseline and comparison

Acceptance Criteria

• Streaming activates when Next.js is disabled and backend returns 2xx
• Client receives first body bytes after first processed chunk
• Peak memory per request constant (chunk buffer + parser state)
• No regressions on static, auction, or discovery endpoints
• Buffered fallback works correctly when post-processors are registered
• StreamingBody abort-on-drop for mid-stream errors

Rollback

Revert Tasks 6-8 commits → buffered behavior restored. Phase 1 improvements kept.

Plan

See docs/superpowers/plans/2026-03-25-streaming-response.md (Tasks 6-10)

[aram356]

Replaced by individual task issues (#573-#577) under epic #563.