Function parse_reference 

pub fn parse_reference(input: &str) -> IResult<&str, Arc<Reference>>

Parse a reference chain: identifier ('.' identifier){0,3}.

Supports up to four segments and uses the global FunctionRegistry to distinguish between:

• plain references (e.g. foo.bar)
• function/method calls (e.g. foo() or foo.bar()), which are handled by higher-level postfix parsing.

Behavior highlights:

• A bare _ is rejected as an invalid reference.
• If the first identifier is a known function and is immediately followed by (, parsing fails so that the caller can treat it as a function call.
• For chained references, if a later segment is a known function and is followed by (, the parser “rolls back” to the shorter reference so that constructs like object.method() are parsed as a reference to object followed by a method call.

Errors are reported via the nom::IResult error variants; resolution-time errors (e.g. undefined reference) occur later when evaluating.