feat(marmot): accept PrivateMessage commits (handshake ratchet + dispatch)

MDK-core (openmls) defaults group configuration to
`MIXED_CIPHERTEXT_WIRE_FORMAT_POLICY` — outgoing commits / proposals
ship as PrivateMessage. Quartz only handled PrivateMessage for
ContentType.APPLICATION; any handshake message came in via the
application ratchet, consumed the first application generation on
whatever sender sent an app message next, and then died with
`Generation 0 already consumed` the moment the receiver saw the real
PrivateMessage commit. That's why every A-side processing of B's
rename / promote / remove / leave commit timed out.

Fix:
  1. SecretTree grows a parallel `handshakeKeyNonceForGeneration`
     path, using the per-sender `handshakeSecret` / `handshakeGeneration`
     the struct already tracked — with its own skipped-keys cache and
     replay-detection set so handshake and application generations
     never clash.
  2. `MlsGroup.decrypt()` dispatches on the PrivateMessage's
     `content_type` BEFORE consuming any ratchet — COMMIT and PROPOSAL
     take the handshake path; APPLICATION stays on the existing
     application path.
  3. COMMIT bodies decode as `Commit || signature<V> || confirmation_tag<V>
     || padding` (RFC 9420 §6.3.1) and route into `processCommit` inline,
     so the caller just sees the epoch advance — no new public API.
     PROPOSAL still rejects (standalone proposals aren't used by
     Marmot flows yet) with a clear message instead of a cryptic
     generation-consumed error.

https://claude.ai/code/session_016kAxdp6ubB5CnF9URhCEzP

tools/marmot-interop/headless/patches/whitenoise-mdk-plaintext-policy.patch

@@ -0,0 +1,11 @@
+--- a/../../../.cargo/git/checkouts/mdk-7d5a3a2420b194f5/8a8d06c/crates/mdk-core/src/groups.rs
++++ b/../../../.cargo/git/checkouts/mdk-7d5a3a2420b194f5/8a8d06c/crates/mdk-core/src/groups.rs
+@@ -1215,7 +1215,7 @@
+         );
+         let group_config = MlsGroupCreateConfig::builder()
+             .ciphersuite(self.ciphersuite)
+-            .wire_format_policy(MIXED_CIPHERTEXT_WIRE_FORMAT_POLICY)
++            .wire_format_policy(MIXED_PLAINTEXT_WIRE_FORMAT_POLICY)
+             .use_ratchet_tree_extension(true)
+             .capabilities(capabilities)
+             .with_group_context_extensions(extensions)