Function normalize_username 

pub fn normalize_username(username: &str) -> Result<Vec<u8>, IdentityError>

Normalise a username and return its UTF-8 bytes.

Applies ASCII lowercasing (not Unicode lowercasing, to avoid the Turkish İ problem) and validates that every character is in [a-z0-9.]. The normalised form must be 1–32 bytes inclusive.

Dot placement rules (enforced after character validation):

• No leading dot (".alice" is rejected).
• No trailing dot ("alice." is rejected).
• No consecutive dots ("alice..1" is rejected).

These rules match the on-chain People pallet’s username validity checks and prevent storage key confusion from structurally malformed names.