fix(q): reject non-response frames

[belowzeroff]

Summary:

• require q_exchange to receive Q response frames (msgtype 2)
• reject async/sync frame headers returned on the client response path
• add a socketpair regression self-test that exercises the bad msgtype path

Tests:

• make lint
• bash -n test/run.sh
• git diff --check
• make test Q_BINARY=/nonexistent

Note: real-q interop remains skipped locally because the available q install cannot connect to its license daemon.