fix(marmot): align MIP compliance tests with held-back v2 and signed commits

Two drifts surfaced as 7 failing tests on the jvmTest run:

1. MarmotGroupData CURRENT_VERSION is held at 2 for mdk-core interop
   (see the const-doc block), but three tests still assumed v3:
   - marmotGroupData_defaultVersionIsThree asserted the constant directly.
   - marmotGroupData_roundTripWithDisappearingSecs and
     buildGroupEvent_appendsExpirationTagWhenDisappearingConfigured relied
     on the default version emitting the v3-only disappearing_message_secs
     field, which the encoder correctly skips for v2.

   Rename the first to `currentVersionIsHeldAtTwoForMdkInterop` with a
   spec-cross-reference comment and an extra assertion that v3 is still
   in SUPPORTED_VERSIONS. Pin version=3 explicitly in the other two so
   they exercise the v3 path regardless of the interop hold-back.

2. MlsGroup.processCommit now requires a non-empty FramedContentTBS
   signature on non-external commits (per RFC 9420 §6.1, introduced in
   0c970945). Four pipeline tests still called the raw processCommit
   overload with signature=ByteArray(0), so they all failed with
   "FramedContentTBS signature missing on commit from leaf 0".

   Add a `processFramedCommit` wrapper on MlsGroupManager that mirrors
   MlsGroup.processFramedCommit + the retention/persistence bookkeeping
   from processCommit, and switch the four tests to feed framedCommitBytes
   through it. MarmotInboundProcessor is unaffected (it already has the
   decoded PublicMessage fields).

All 7 originally-failing Marmot tests now pass; the 4 unrelated
NostrClient network tests remain independently flaky.

https://claude.ai/code/session_01XQNAmwn1y87GAoK94QWgyn

quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/marmot/MarmotMipComplianceTest.kt

@@ -60,8 +60,17 @@ class MarmotMipComplianceTest {
     // ---------------------------------------------------------------------- MIP-01
 
     @Test
-    fun marmotGroupData_defaultVersionIsThree() {
-        assertEquals(3, MarmotGroupData.CURRENT_VERSION)
+    fun marmotGroupData_currentVersionIsHeldAtTwoForMdkInterop() {
+        // MIP-01 spec targets v3, but mdk-core (whitenoise-rs' MLS engine)
+        // rejects v3 NostrGroupData payloads with `ExtensionFormatError`
+        // ("Trailing bytes in NostrGroupDataExtension"), violating the spec's
+        // forward-compat rule. Until mdk-core ships the fix, Amethyst holds
+        // CURRENT_VERSION at 2 for cross-client interop; v3 stays parseable
+        // (see disappearing_message_secs round-trip test below). Bump this
+        // assertion back to 3 in lockstep with the MarmotGroupData constant
+        // once mdk publishes the parser fix.
+        assertEquals(2, MarmotGroupData.CURRENT_VERSION)
+        assertTrue(3 in MarmotGroupData.SUPPORTED_VERSIONS, "v3 MUST remain parseable for forward compat")
     }
 
     @Test
@@ -89,8 +98,13 @@ class MarmotMipComplianceTest {
 
     @Test
     fun marmotGroupData_roundTripWithDisappearingSecs() {
+        // disappearing_message_secs is a v3+ field. CURRENT_VERSION is held
+        // at 2 for MDK interop (see marmotGroupData_currentVersionIsHeldAtTwoForMdkInterop),
+        // so this test pins v3 explicitly to exercise the v3 encoder/decoder
+        // path end-to-end.
         val original =
             MarmotGroupData(
+                version = 3,
                 nostrGroupId = groupId32,
                 adminPubkeys = listOf(adminPubkey),
                 relays = listOf("wss://relay.example/"),