Implement streaming response optimization for non-Next.js publisher proxy

[aram356]

Context

The publisher proxy currently buffers the entire response body in memory before sending any bytes to the client. For a 222KB HTML page, peak memory is ~4x the response size and no bytes reach the client until all processing completes.

Performance results (staging vs production, median over 5 runs, Chrome 1440x900)

Metric	Production (v135, buffered)	Staging (v136, streaming)	Delta
TTFB	54 ms	35 ms	-19 ms (-35%)
First Paint	186 ms	160 ms	-26 ms (-14%)
First Contentful Paint	186 ms	160 ms	-26 ms (-14%)
DOM Content Loaded	286 ms	282 ms	-4 ms (~same)
DOM Complete	1060 ms	663 ms	-397 ms (-37%)

Measured on getpurpose.ai. Production (v135) buffers the entire response before sending. Staging (v136) streams processed chunks incrementally via StreamingBody.

Spec

See streaming response design spec (PR #562).

Plan

See implementation plan (PR #562).

Phase 1: Make streaming pipeline chunk-emitting (PR #583)

Ships independently with immediate memory savings.

• Phase 1, Task 1: Fix encoder finalization in process_through_compression #568 — Phase 1, Task 1: Fix encoder finalization in process_through_compression
• Phase 1, Task 2: Convert process_gzip_to_gzip to chunk-based processing #569 — Phase 1, Task 2: Convert process_gzip_to_gzip to chunk-based processing
• Phase 1, Task 3: Convert decompress_and_process to chunk-based processing #570 — Phase 1, Task 3: Convert decompress_and_process to chunk-based processing
• Phase 1, Task 4: Rewrite HtmlRewriterAdapter for incremental streaming #571 — Phase 1, Task 4: Rewrite HtmlRewriterAdapter for incremental streaming
• Phase 1, Task 5: Full verification #572 — Phase 1, Task 5: Full verification

Phase 2: Stream responses to client via StreamingBody (PR #585)

Depends on Phase 1. Adds TTFB/TTLB improvement.

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

Phase 3: Make script rewriters fragment-safe (PR #591)

Depends on Phase 2. Removes the buffered fallback, enabling full streaming even with GTM/NextJS script rewriters active.

• Phase 3, Task 11: Make NextJsNextDataRewriter fragment-safe #586 — Phase 3, Task 11: Make NextJsNextDataRewriter fragment-safe
• Phase 3, Task 12: Make GoogleTagManagerIntegration rewrite fragment-safe #587 — Phase 3, Task 12: Make GoogleTagManagerIntegration rewrite fragment-safe
• Phase 3, Task 13: Remove buffered mode from HtmlRewriterAdapter #588 — Phase 3, Task 13: Remove buffered mode from HtmlRewriterAdapter
• Phase 3, Task 14: Always use streaming adapter in create_html_processor #589 — Phase 3, Task 14: Always use streaming adapter in create_html_processor
• Phase 3, Task 15: Full verification and regression tests #590 — Phase 3, Task 15: Full verification and regression tests

Phase 4: Stream binary pass-through responses

Depends on Phase 2. Non-processable content (images, fonts, video) currently buffers in memory unnecessarily. Phase 4 streams them directly via io::copy into StreamingBody.

• Phase 4, Task 16: Stream binary pass-through responses via io::copy #592 — Phase 4, Task 16: Stream binary pass-through responses via io::copy
• Phase 4, Task 17: Binary pass-through tests and verification #593 — Phase 4, Task 17: Binary pass-through tests and verification

Acceptance Criteria

• Streaming activates for all 2xx responses (text and binary)
• Peak memory per request reduced from ~4x to constant (chunk buffer + parser state)
• Client receives first body bytes after first processed chunk, not after full buffering
• No regressions on static, auction, or discovery endpoints
• Buffered fallback for HTML with post-processors and non-2xx error pages
• Script rewriters (GTM, NextJS) work correctly under streaming fragmentation
• Binary responses (images, fonts) stream via pass-through without processing overhead