Reverts the SelfRemove-as-commit approach. openmls rejects a
commit whose only proposal is a SelfRemove from the committer with
RequiredPathNotFound — the spec model is that the removing member
publishes a STANDALONE proposal, and an admin receiver
(wn's mdk auto-commit path) folds it into their next commit.
Add `MlsGroup.buildSelfRemoveProposalMessage()` which frames
Proposal.SelfRemove as `MlsMessage(PublicMessage{proposal})` with
the proper FramedContent / signature / membership_tag and returns
the ready-to-publish bytes + the pre-commit exporter key for outer
encryption. Rewire `MlsGroupManager.leaveGroup` and
`MarmotManager.leaveGroup` to use it.
Target: test 11 (amy leave → B auto-commits SelfRemove →
removes amy from member list).
https://claude.ai/code/session_1469d7f4-bb66-4ffa-a44d-1dfa4b526484
Claude
72d1793a25