fix(marmot): send SelfRemove as standalone PublicMessage proposal

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

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/marmot/MarmotManager.kt

@@ -290,16 +290,16 @@ class MarmotManager(
      * Returns proposal bytes to publish (as a GroupEvent).
      */
     suspend fun leaveGroup(nostrGroupId: HexKey): OutboundGroupEvent {
-        // leaveGroup() builds a SelfRemove-only commit (MIP-03 non-admin
-        // exception) and removes local state. It must run BEFORE we tear
-        // down the subscriptions so the pre-commit exporter key is still
-        // reachable for outer encryption.
-        val commitResult = groupManager.leaveGroup(nostrGroupId)
+        // leaveGroup() returns the framed standalone SelfRemove proposal
+        // (PublicMessage{Proposal}) plus the pre-commit exporter key for
+        // outer encryption. Runs BEFORE we tear down the subscriptions so
+        // the pre-commit exporter is still derivable.
+        val (framedBytes, exporterKey) = groupManager.leaveGroup(nostrGroupId)
         val outboundEvent =
             outboundProcessor.buildCommitEvent(
                 nostrGroupId = nostrGroupId,
-                commitBytes = commitResult.framedCommitBytes,
-                exporterKey = commitResult.preCommitExporterSecret,
+                commitBytes = framedBytes,
+                exporterKey = exporterKey,
             )
 
         subscriptionManager.unsubscribeGroup(nostrGroupId)