Add typed X-DNS-Prefetch-Control response metadata

[fewensa]

Summary

• add bounded, policy-free X-DNS-Prefetch-Control parsing
• expose typed client and server response accessors and facade exports
• preserve raw metadata on parsing failures and reject duplicates/unsupported values

Verification

• cargo fmt --all -- --check
• git diff --check
• cargo test -p rttp-protocol x_dns_prefetch_control
• cargo test -p rttp_client x_dns_prefetch_control_metadata
• cargo test -p rttp-server x_dns_prefetch_control
• cargo test -p rttp --features client x_dns_prefetch_control --no-run

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