Proposal: Module backwards compatibility

[mpvl]

Proposal #2330 relies on MVS, which requires a notion of backwards compatibility. This is discussed in this sub-proposal:
https://github.com/cue-lang/proposal/blob/main/designs/modules/2451-modules-compat.md

[myitcv]

This proposal was superseded by #2939. As we updated in #2939 (comment), that proposal (and its sub-proposals) have now been accepted, hence we are closing this discussion to represent the decision on the v3 proposal.