feat(battle-node): add OutboundSequencer.Clear() for terminate cascade

Audit Md11 (part 2 of 2). Adds an explicit Clear() so BattleSession can
release the archive at battle-end instead of waiting for the participant to
be GC'd. _next is intentionally NOT reset — a post-Clear emit is a bug per
the design, but the seq stream must stay monotonic if it does happen.

Tests cover empty archive after Clear, _next preservation across Clear,
and Clear-on-empty no-op. The BattleSession integration that calls Clear
lands in the next commit.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.BattleNode/Reliability/OutboundSequencer.cs

@@ -24,4 +24,13 @@ public sealed class OutboundSequencer
 
     public MsgEnvelope WrapNoStock(MsgEnvelope envelope) =>
         envelope with { PlaySeq = null };
+
+    /// <summary>
+    /// Drop all archived envelopes. Called from BattleSession's terminate cascade so
+    /// the archive — the heavy state — is released the moment the battle ends, rather
+    /// than waiting for the participant to be GC'd. <c>_next</c> is left untouched:
+    /// a participant emitting after Clear is a bug, not a recovery case, but the seq
+    /// stream stays monotonic so a stray emit doesn't silently re-use a playSeq value.
+    /// </summary>
+    public void Clear() => _archive.Clear();
 }