Phase 3, Task 15: Full verification and regression tests

[aram356]

Parent: #563
Depends on: Tasks 11-14

Summary

Verify all changes and add regression tests with small chunk sizes.

Checklist

• `cargo test --workspace`
• `cargo clippy --workspace --all-targets --all-features -- -D warnings`
• `cargo fmt --all -- --check`
• `cd crates/js/lib && npx vitest run`
• `cargo build --package trusted-server-adapter-fastly --release --target wasm32-wasip1`
• Regression: small-chunk NextJS `NEXT_DATA` test passes
• Regression: small-chunk inline GTM script test passes
• Existing nextjs and GTM integration tests pass unchanged

Plan

See `docs/superpowers/specs/2026-03-25-streaming-response-design.md` — Phase 3