Raised when read-only replay reaches a journal write.
Continuum.Replay and mix continuum.replay are structurally read-only: the
context they build is given its history up front and carries
Continuum.Runtime.Journal.ReadOnly, which implements every journal callback
as a raise. Reaching one means a replay path tried to mutate durable state,
so the raise is a bug report rather than something a caller should rescue.