stateless-me/uuidv47: ⚡ UUIDv47 = v4 privacy + v7 performance

UUIDv47 is a C library that combines UUIDv7's database performance benefits with UUIDv4's privacy features. It stores sortable UUIDv7 internally for better database indexing while presenting UUIDv4-looking identifiers externally through SipHash-masked timestamps. The library provides deterministic, invertible mapping with key-recovery resistance and maintains RFC compatibility.