Add temporary JA4/TLS debug endpoint

[prk-Jr]

Summary

• Adds a temporary GET /_ts/debug/ja4 endpoint to the Fastly adapter to capture JA4 TLS fingerprint, H2 fingerprint, cipher suite, TLS version, user-agent, and Client Hints in production
• Exposes these signals to evaluate TLS fingerprinting as a cross-browser identity seed and OpenRTB ext validated-browser signal
• Endpoint is marked temporary and should be removed after evaluation

Changes

File	Change
crates/trusted-server-adapter-fastly/src/main.rs	Added build_ja4_debug_response handler and route; added unit tests

Closes

Closes #645

Test plan

• cargo test --workspace
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• JS tests: cd crates/js/lib && npx vitest run
• JS format: cd crates/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: Deploy and hit /_ts/debug/ja4 to confirm fingerprint data returns

Checklist

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