docs(net): N6 accepted - Opus review PASS; five owed register rows filed

The final slice review verified every retail address claim down to the
three distinct gate strictness masks (0x41 strict for NAK/handshake, no-ZF
>= for the 5 s sweep) and found no handshake, eviction, or ring defect.
This acceptance settles the campaign's remaining bookkeeping debt the
review surfaced: TS-58 (no TimeSync/Echo keepalive), TS-59 (no Flow
report), TS-60 (no 140 s dead-link/referral), TS-61 (send-failure burns
sequence+key), and AP-126 (one monotonic clock) are now real register
rows instead of dangling citations in shipped code. DropAll additionally
resets the completed-sequence ring (INFO-4's latent session-reset trap),
and the ledger corrects the post-acceptance retry-drop attribution to
NetworkManager's pre-route (INFO-5). N6 SHA f9c5e47e and its revert line
recorded. Core.Net 757/757 green after the ring-reset change.

Campaign N's implementation is complete: N0-N6 all shipped, all reviewed.
The remaining acceptance is the user Coldeve endurance session.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.Core.Net/Packets/FragmentAssembler.cs

@@ -277,8 +277,20 @@ public sealed class FragmentAssembler
         return evicted;
     }
 
-    /// <summary>Discard all in-flight partial messages.</summary>
-    public void DropAll() => _inFlight.Clear();
+    /// <summary>
+    /// Discard all in-flight partial messages AND the completed-sequence
+    /// ring. The ring reset closes a latent trap (N6 review INFO-4): a
+    /// future session-reset caller would otherwise retain completed marks
+    /// while ACE restarts its fragment sequence at 0, false-dropping the
+    /// new session's first messages — exactly the failure class the ring's
+    /// zero-slot guard exists to prevent.
+    /// </summary>
+    public void DropAll()
+    {
+        _inFlight.Clear();
+        _completedNext = 0;
+        _completedCount = 0;
+    }
 
     private bool WasRecentlyCompleted(uint sequence)
     {