Per-line option/key split
Each line is split at the sshd-defined boundary: an optional leading run of comma-separated option tokens, then whitespace, then the key type (ssh-rsa, ssh-ed25519, ecdsa-sha2-*, sk-ecdsa-*, etc.), the base64 key blob, and an optional trailing comment. Quoted option values like command="..." are parsed with quote-aware splitting so commas embedded inside a command string aren't mistaken for option separators.