Validate a blockchain address. Supply `chain` to check against a specific chain, or omit it to auto-detect. Ethereum addresses report their EIP-55 checksum state; Bitcoin covers base58check (P2PKH/P2SH) and bech32/bech32m segwit (P2WPKH/P2WSH/P2TR). A well-formed but invalid address returns `valid:false` — not an error.