feat(routing): preserve response metadata fidelity

[MichalCz]

Summary

• Preserve status text, ordered headers, and Latin-1 response metadata across Host and Guest runtimes.
• Reject invalid local HTTP ByteString headers and status text at Broker and Guest boundaries; malformed remote metadata remains a protocol error.
• Correlate response and error envelopes with request IDs and harden terminal error handling.
• Document consumer-visible HTTP and Fetch compatibility boundaries separately.

Small breaking change

• Invalid local header input that previously surfaced later as protocol-error now rejects at the API boundary (TypeError in Node/Bun, ValueError in Python). Unhandled Guest response validation becomes local-handler-failure.
• Valid Latin-1 values and malformed remote-metadata handling are unchanged. The compatibility impact is small: update error handling rather than retrying invalid input.

Validation

• npm test
• npm run lint
• npm --workspace=@signicode/verser2-guest-bun run test
• npm --workspace=@signicode/verser2-guest-python run test
• npm --workspace=@signicode/verser2-guest-python run lint

[MichalCz]

Review disposition: retain the strict shared decoder. Replacing it with Fetch Headers would lose repeated-value/strict-shape fidelity and Python parity; the native-validator wrapper refactor is not justified for this PR.