fix(marmot): sign commits under pre-proposal extensions (fix InvalidMembershipTag)

For a GroupContextExtensions commit, openmls signs the
`FramedContentTBS` over `group.public_group.group_context()` — the
UNMUTATED context, before the `diff` applies the GCE proposal. The
post-proposal extensions only make it into the HPKE path-encryption
context and the subsequent key schedule, not into the TBS / signature /
membership_tag.

Quartz's `applyProposal` handler for `GroupContextExtensions` mutates
`groupContext.extensions` inline — by the time `commit()` captured
`preCommitContextBytes`, the extensions were already updated. The
signature and membership_tag were then minted over the post-GCE context,
and openmls's `verify_membership` recomputed the MAC over the pre-GCE
context and reported `ValidationError(InvalidMembershipTag)` on every
cross-impl rename / promote / demote / leave.

Fix: snapshot `groupContext.extensions` before any proposal is applied,
and rebuild `preCommitContextBytes` with those original extensions. The
path-encryption context and post-commit key schedule still use the
updated extensions, matching openmls.

https://claude.ai/code/session_016kAxdp6ubB5CnF9URhCEzP

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/marmot/mls/group/MlsGroup.kt

@@ -411,6 +411,15 @@ class MlsGroup private constructor(
         val preCommitExporterSecret =
             exporterSecret("marmot", "group-event".encodeToByteArray(), 32)
 
+        // Snapshot the pre-proposal extensions. GroupContextExtensions proposals
+        // mutate `groupContext.extensions` the moment they're applied, but
+        // openmls signs the commit's FramedContentTBS over the UNMUTATED
+        // context (the one in `public_group` before `diff` applies proposals).
+        // Without this snapshot, a GCE commit on the quartz side uses the new
+        // extensions for TBS + membership_tag, and openmls rejects it with
+        // `ValidationError(InvalidMembershipTag)`.
+        val preCommitExtensions = groupContext.extensions
+
         val proposalOrRefs = proposals.map { ProposalOrRef.Inline(it.proposal) }
 
         // Check if we need an UpdatePath. RFC 9420 §12.4.1: the path value
@@ -601,8 +610,10 @@ class MlsGroup private constructor(
         // is sent — the one we're about to leave. Receivers (openmls/mdk)
         // strict-verify both, so we must use the leaf signing key that's
         // still in the pre-commit tree and the membership_key derived from
-        // the pre-commit epoch secrets.
-        val preCommitContextBytes = groupContext.toTlsBytes()
+        // the pre-commit epoch secrets. Extensions need explicit rewind to
+        // pre-proposal state for GroupContextExtensions commits.
+        val preCommitContextBytes =
+            groupContext.copy(extensions = preCommitExtensions).toTlsBytes()
         val preCommitMembershipKey = epochSecrets.membershipKey
         val preCommitSigningKey = signingPrivateKey