#135807 makes the empty-transcript refusal emit its attempt record through
the shared-metric lifecycle (on main that exit recorded nothing), but
`empty_transcript` was not in the closed `COMPRESSION_FAILURE_CLASSES` set,
so the `hermes.compression.count` row read `failed / other`. Add the class
to the contract, the v4 package schema enum and the metric docs so the row
says what happened.
760c93fef7