fix: Panic in write_utf8 (assert s.len() < 64)

[keepsimple1]

This is to fix issue #483

The fix replaces the panic with graceful error handling by threading a Result through the write path:

• New WriteError enum and WriteResult alias.
• DnsRecordExt::write, write_question, write_record, write_name, write_utf8 all return WriteResult instead of panicking / returning bool.