Function classify_call 

fn classify_call(tokens: &[TokenTree]) -> Option<LowerCall>

Classify a flat token slice as a lowerable top-level call.

Returns None for anything that’s not a lowerable call.

Supported patterns:

• ident(args…) — simple call, 2 tokens
• ident.ident.…(args…) — R dot-name call (e.g. is.null), multiple tokens + group
• pkg::fn(args…) — namespaced call, 5 tokens