roadmap: add MLS (RFC 9420) for E2E group call encryption

- FC-P5-T5 added to task plan with full design notes
- OpenMLS/TreeKEM approach: O(log n) key rotation, forward secrecy
- Current group calls marked as transport-encrypted only (QUIC)
- UI warning shown when starting group call
- Updated completed task statuses (read receipts, wrapping, tab complete)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

warzone/crates/warzone-server/src/routes/web.rs

@@ -1724,6 +1724,7 @@ async function startGroupCall() {
   }).catch(() => {});
 
   addSys('Group call started in #' + gname + ' \u2014 waiting for others to join');
+  addSys('\u{26A0}\u{FE0F} Group calls are transport-encrypted (QUIC), not E2E encrypted');
   await joinGroupCallAudio();
 }