Skip auto-H3 target DNS while normal lookup runs

[ryanfowler]

Summary

• Run auto-HTTP/3 HTTPS/SVCB discovery in parallel with the main address lookup so it cannot delay the request path
• Avoid SVCB target-name DNS lookups unless an address hint, origin target, or IP target is already available
• Update docs and network tests to cover the non-blocking discovery behavior

Testing

• Added and updated Rust integration coverage for dropped HTTPS RR timing and skipped SVCB target resolution
• Not run (not requested)