Flatten outer protocol dispatch in packet_matching demo

Match arms with block bodies are mis-lowered by the codegen: arm-local
`var` declarations have their initializers dropped, and `if/else` inside
an arm block gets flattened so the gating condition disappears. The
result is undefined-value reads and incorrect protocol routing.

Signed-off-by: Cong Wang <cwang@multikernel.io>

Author: congwang-mk