URL record type with scheme, username, password, host, port, path, query, fragment. State-machine parser handling special schemes (http, https, ftp, ws, wss, file) with default ports. Host parsing for domains, IPv4 (decimal/hex/octal), and IPv6 (including IPv4-mapped and :: compression). Percent-encoding/decoding per spec encode sets (C0, fragment, query, path, userinfo). Relative URL resolution via base URL. URL serialization with round-trip fidelity. Origin derivation (tuple for special schemes, opaque otherwise). 88 tests covering absolute URLs, scheme handling, host types, IPv4/IPv6 parsing and serialization, percent encoding, path dot resolution, relative URL resolution, serialization round-trips, file URLs, and edge cases. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>