Core: Report an unknown name in an operand as undefined

get_typed_operand fell through its Name branch for a name that is not a
local, a global or a vmlinux constant, and raised the generic
"Unsupported operand type" meant for unknown node kinds. Since return
goes through it, forgetting `from vmlinux import XDP_PASS` produced that
message; it says "Undefined variable XDP_PASS" now, like every other
read.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: r41k0u

pythonbpf/expr/expr_pass.py

@@ -254,6 +254,7 @@ def get_typed_operand(func, compilation_context, operand, builder, local_sym_tab
             vmlinux_result = VmlinuxHandlerRegistry.handle_name(operand.id)
             if vmlinux_result is not None:
                 return vmlinux_result  # (i64 constant, its C rank)
+            raise SyntaxError(f"Undefined variable {operand.id}")
     elif isinstance(operand, ast.Constant):
         if isinstance(operand.value, (int, bool)):
             v = int(operand.value)