Function parse 

pub fn parse(source: &str) -> Result<Ast, ParseError>

Parse an RSQL/FIQL filter string into an unvalidated AST.

Two-pass pipeline:

1. lexer::tokenize — semantic scan → Vec<Spanned> (literals typed at lex time)
2. grammar::parse_tokens — recursive descent on the token stream → Ast

On failure, returns ParseError::At with line, column, source snippet, and a caret underline — ready to display directly to API consumers.