Phase 1: Make streaming pipeline chunk-emitting

[aram356]

Summary

Make the internal streaming pipeline truly chunk-emitting so processors emit output incrementally instead of buffering entire documents. This is the foundation for Phase 2 (streaming responses to the client via StreamingBody) but ships independently with immediate memory and correctness benefits.

Closes #568, closes #569, closes #570, closes #571, closes #572.
Part of epic #563.

What changed

Encoder finalization fix (process_through_compression):

• Changed signature from W (by value) to &mut W so callers retain ownership
• Replaced flush() + drop(encoder) with explicit encoder.finish() / encoder.into_inner()
• Finalization errors now propagate instead of being silently swallowed
• Fixes pre-existing correctness issue for deflate/brotli, prevents regression when gzip moves to this path

All compression paths now chunk-based:

• process_gzip_to_gzip: replaced read_to_end() with process_through_compression pattern
• decompress_and_process (used by *_to_none paths): replaced read_to_end() with chunk read loop
• All 6 compression paths now follow the same structure: read chunk → process → write → repeat

Unified process_chunks method:

• Extracted shared chunk read/process/write loop from process_uncompressed, process_through_compression, and decompress_and_process into a single process_chunks method
• Reduces 3 near-identical implementations to 1

HtmlRewriterAdapter incremental streaming:

• lol_html rewriter created eagerly in constructor with Rc<RefCell<Vec<u8>>> shared output sink
• process_chunk() now emits output on every call, not just is_last
• Adapter is single-use (reset() is a no-op — Settings consumed by constructor)

HtmlWithPostProcessing adaptation:

• Added output accumulation buffer for the is_last chunk when post-processors are registered
• Intermediate chunks pass through directly; final chunk triggers post-processing on accumulated output
• When no post-processors are registered (the streaming path), it's a pure passthrough

Files changed

File	Lines	What
streaming_processor.rs	+380 -329	All pipeline changes above
html_processor.rs	+29 -1	Post-processor accumulation for is_last

Verification

• cargo test --workspace — 748 passed, 0 failed
• cargo clippy --workspace --all-targets --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean
• npx vitest run — 282 passed
• cargo build --release --target wasm32-wasip1 — success

Test plan

• Existing streaming_processor tests updated for incremental output
• New deflate round-trip test
• New gzip round-trip test (chunk-based)
• New gzip-to-none test
• HtmlRewriterAdapter per-chunk emission test
• Full pipeline integration test with HTML rewriting
• WASM build succeeds