feat(net): Phase 4.9 — send ACK_SEQUENCE for every received server packet

Root cause of the still-purple-haze symptom AND the ACE-side
"Network Timeout" drop after ~60s. acdream was never sending
acknowledgement packets back to the server, so the server's
reliability layer saw a one-way stream and eventually dropped the
session. During the 60s window the player rendered to other clients
as the stationary purple loading haze (AC's "this client is in
portal-space transition" indicator).

Pattern ported from
references/holtburger/crates/holtburger-session/src/session/
{send.rs::send_ack, receive.rs::finalize_ordered_server_packet}.
The proper holtburger pattern is per-packet acks, NOT a periodic
heartbeat: every received server packet with sequence > 0 and no
ACK_SEQUENCE flag of its own gets a bare control packet sent back
with:

  PacketHeader {
    Flags    = ACK_SEQUENCE (0x4000),
    Sequence = current_client_sequence (= last issued, no increment),
    Id       = session client id,
  }
  Body = u32 little-endian server sequence being acked

Acks are cleartext control packets (no EncryptedChecksum) and
re-use the most recently issued client sequence rather than
consuming a new one — they aren't part of the reliable stream the
server tracks for retransmits.

Wired into ProcessDatagram so both Tick (post-InWorld) and PumpOnce
(during Connect/EnterWorld) trigger acks on every received non-ack
server packet.

Also (per user request) upgrades the CLAUDE.md description of the
holtburger reference repo from "Rust AC client crate" to "almost-
complete Rust TUI AC client — the most authoritative reference for
client-side behavior in the project, look here FIRST for anything
WorldSession or message-builder related." This was the third time
in two days I would have saved hours by checking holtburger first
instead of guessing at the protocol from ACE alone.

220 tests green.

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

src/AcDream.Core.Net/WorldSession.cs

@@ -254,6 +254,20 @@ public sealed class WorldSession : IDisposable
         var dec = PacketCodec.TryDecode(bytes, _inboundIsaac);
         if (!dec.IsOk) return;
 
+        // Phase 4.9: send an ACK_SEQUENCE control packet for every received
+        // server packet with sequence > 0 and no ACK flag of its own. This
+        // is the proper holtburger pattern (every received packet gets an
+        // ack queued back; not periodic). Without it, ACE drops the session
+        // with "Network Timeout" because it sees no acks coming back —
+        // which surfaces in other clients' views as the player rendering
+        // as a stationary purple haze (loading state).
+        var serverHeader = dec.Packet!.Header;
+        if (serverHeader.Sequence > 0
+            && (serverHeader.Flags & PacketHeaderFlags.AckSequence) == 0)
+        {
+            SendAck(serverHeader.Sequence);
+        }
+
         foreach (var frag in dec.Packet!.Fragments)
         {
             var body = _assembler.Ingest(frag, out _);
@@ -335,6 +349,53 @@ public sealed class WorldSession : IDisposable
         _net.Send(datagram);
     }
 
+    /// <summary>
+    /// Phase 4.9: send a bare ACK_SEQUENCE control packet acknowledging
+    /// <paramref name="serverPacketSequence"/>. This is a cleartext control
+    /// packet (no EncryptedChecksum) — the body is just the 4-byte server
+    /// sequence number being acknowledged. The header re-uses the most
+    /// recently sent client sequence (no increment) because acks aren't
+    /// themselves part of the reliable stream the server tracks.
+    ///
+    /// <para>
+    /// Without sending these, ACE drops the session with
+    /// <c>Network Timeout</c> after ~60s — and during that 60s the
+    /// character appears to other clients as a stationary purple haze
+    /// (loading state) because the server hasn't seen the client confirm
+    /// any post-EnterWorld traffic.
+    /// </para>
+    ///
+    /// <para>
+    /// Pattern ported from
+    /// <c>references/holtburger/crates/holtburger-session/src/session/send.rs::send_ack</c>
+    /// and the receive-side trigger at
+    /// <c>.../session/receive.rs::finalize_ordered_server_packet</c>.
+    /// </para>
+    /// </summary>
+    private void SendAck(uint serverPacketSequence)
+    {
+        // 4-byte body: little-endian u32 of the server sequence we're acking.
+        Span<byte> body = stackalloc byte[4];
+        BinaryPrimitives.WriteUInt32LittleEndian(body, serverPacketSequence);
+
+        // Holtburger uses current_client_sequence (= packet_sequence - 1) for
+        // ack headers. We mirror that — acks borrow the most recently issued
+        // client sequence rather than consuming a new one.
+        uint ackHeaderSequence = _clientPacketSequence > 0
+            ? _clientPacketSequence - 1
+            : 0u;
+
+        var header = new PacketHeader
+        {
+            Sequence = ackHeaderSequence,
+            Flags    = PacketHeaderFlags.AckSequence,
+            Id       = _sessionClientId,
+        };
+
+        byte[] datagram = PacketCodec.Encode(header, body, outboundIsaac: null);
+        _net.Send(datagram);
+    }
+
     private void Transition(State next)
     {
         if (CurrentState == next) return;