Function normalize_raw_locale 

pub fn normalize_raw_locale(raw: &str) -> String

Normalizes a raw locale string for BCP47 parse.

• Strips encoding suffix (.UTF-8, .utf8)
• Strips @modifier (e.g. @euro)
• Converts _ separators to -
• Trims whitespace

Does not treat C / POSIX / C.UTF-8 as English — those parse as invalid/unsupported and fall through negotiation.