Lin-mod immutability was implemented in T119236 (see task for details of what this means), but is currently only enforced in tests and debug mode.

To catch more errors and enforce data integrity we should consider enforcing it all the time.

Some preliminary timing suggests there is no noticeable performance regression, but this should be checked more rigorously.

We may also want to consider a warning mode to begin with, where modification of an immutable results in a logged warning, rather than an exception.