Make body hold parser-aware and stream Next.js processing

[prk-Jr]

Summary

• Use a parser-generated, request-specific seam to delay only the structural </body> tail, preventing inline JavaScript or JSON literals from holding most of the page.
• Replace whole-document Next.js post-processing with bounded per-document streaming that rewrites complete RSC groups and restores malformed, incomplete, or over-limit payloads unchanged.
• Preserve auction ordering and terminal telemetry across streaming, compressed, missing-body-end, and failure paths.

Changes

File	Change
crates/trusted-server-core/src/html_processor.rs	Insert a parser-confirmed deferred body marker and stream transformed output.
crates/trusted-server-core/src/integrations/google_tag_manager.rs	Adapt registration to the per-document processor interface.
crates/trusted-server-core/src/integrations/mod.rs	Define the per-document HTML stream processor factory contract.
crates/trusted-server-core/src/integrations/nextjs/html_post_process.rs	Remove the whole-document Next.js post-processor.
crates/trusted-server-core/src/integrations/nextjs/mod.rs	Register the bounded Next.js stream processor.
crates/trusted-server-core/src/integrations/nextjs/rsc.rs	Classify malformed and incomplete RSC T-chunks safely, including decoded character boundaries.
crates/trusted-server-core/src/integrations/nextjs/rsc_placeholders.rs	Capture RSC scripts into request-namespaced placeholders with bounded fallback.
crates/trusted-server-core/src/integrations/nextjs/rsc_stream.rs	Add ordered, bounded RSC grouping, rewriting, restoration, and streaming release.
crates/trusted-server-core/src/integrations/nextjs/script_rewriter.rs	Move Next.js script fragment state to the document scope.
crates/trusted-server-core/src/integrations/nextjs/shared.rs	Share URL rewrite support with the stream processor.
crates/trusted-server-core/src/integrations/registry.rs	Construct processor instances per document.
crates/trusted-server-core/src/publisher.rs	Collect auctions at the parser-confirmed seam while preserving early output and abandonment telemetry.
docs/guide/integrations/nextjs.md	Document bounded Next.js streaming and unchanged fallback behavior.
docs/superpowers/specs/2026-09-07-850-parser-aware-body-hold-nextjs-streaming-design.md	Add the reviewed design specification.
docs/superpowers/plans/2026-09-07-850-parser-aware-body-hold-nextjs-streaming.md	Add the reviewed implementation plan.

Closes

Closes #850

Test plan

• cargo test-fastly && cargo test-axum
• cargo clippy-fastly && cargo clippy-axum
• cargo fmt --all -- --check
• JS tests: cd crates/trusted-server-js/lib && npx vitest run (blocked by the existing CommonJS/ESM incompatibility between html-encoding-sniffer and @exodus/bytes; no JS files changed)
• JS format: cd crates/trusted-server-js/lib && npm run format
• Docs format: cd docs && npm run format
• WASM build: cargo build --package trusted-server-adapter-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve
• Other: cargo test-cloudflare, cargo test-spin, cargo clippy-cloudflare, cargo clippy-cloudflare-wasm, cargo clippy-spin-native, and cargo clippy-spin-wasm

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — use expect("should ...")
• Uses repository logging macros (not println!)
• New code has tests
• No secrets or credentials committed