test_CMsgRecorderReadsLiveMessage
Regression test for issue #338.
A recorder attached to a C module’s C output message must read the module’s
live message data, not a stale snapshot. The C-message read path reaches the
payload by pointer arithmetic from the start of the {type}_C struct, so this
test verifies end-to-end that the recorded payload tracks the values the module
actually writes over time (and is not constant, which would indicate the
recorder is bound to the wrong / a copied address).