feat(net): N3 - AckNakScheduler, retail 2.0s cumulative ack replaces per-packet acks

Campaign N slice N3. Retail never acks per packet: SharedNet::EnqueuePak
@ 0x00543B10 is the binary's only AckSequence (0x4000) construction site,
gated at >= 2.0 s on ReceiverData::timeStamp_ (@ +0x10), armed at
connection birth by ReceiverData::Init @ 0x00548EF0, and arbitrated
NAK-xor-ack per sweep by ClientNet::ProcessConnection @ 0x00545450
(m_SeqIDsWeNAKed non-empty -> EnqueueNaks, else EnqueuePak;
SharedNet::EnqueueNaks @ 0x00543BD0 shares the SAME timestamp -
campaign landmine #7).

- New Transport/AckNakScheduler: owns the one shared timestamp; a
  non-empty NAK set suppresses the ack (N4 emits RequestRetransmit in
  that branch; in N3 it emits nothing - a documented transitional state,
  safe for exactly one slice on loopback), else ONE cleartext exact-flags
  AckSequence carrying the tracker's HighestIdReceived, header sequence
  borrowed from HighestIdSent without incrementing, 4-byte LE body.
  Flags are an EQUALITY, never an OR (landmine #5 - ACE's dedup
  exemption NetworkSession.cs:342-343 and watermark-skip :474-476 both
  require the exact value).
- ReliableTransport.Sweep pump order per FlowQueue::Empty @ 0x00548A20:
  interval clock, NAK/ack arbitration, pending resends, prune. The sweep
  already runs in Tick and both handshake pump loops (landmine #8), so
  cumulative acks flow during the character-list/enter-world floods at
  ACE's own ~2 s cadence.
- WorldSession: the Phase 4.9 per-packet reflex ack in ProcessDatagram
  and SendAck are DELETED; the [net-tick] acks/s probe now reads
  Stats.AcksSent; new internal TransportClockSource seam drives the
  2.0 s gate on virtual time in the conformance suite.
- N1 Fable-review advisory retired (Time-stamp fold-in): fresh reliable
  sends now stamp Header.Time = the current interval id, matching retail
  FlowQueue::TransmitNewPackets @ 0x00547A60 (header build at
  0x00547A84); resends already re-stamped. ACE never reads inbound
  Header.Time, so the wire stays compatible.

Tests: 723 Core.Net (7 new) - gate cadence + watermark-at-emission,
flags-equality pin + model acceptance at the reused sequence without a
watermark advance, NAK suppression and resume after the gap clears, a
50-packet CreateObject flood collapsing to ONE ack, the quiet-session
keepalive property across a 120 s virtual horizon (the reflex ack's
keepalive role, replaced and proven against ACE's 60 s TimeoutDeadline),
the Time fold-in, and a full FakeAceTransport lifecycle with zero
CRC/state/duplicate drops. Full solution Release: 9,744 passed /
5 skipped / 0 failed. Connected world-lifecycle gate PASS (capped +
uncapped-reconnect, graceful exits, 0 failures); canonical nine-stop
route PASS (0 failures).

Campaign section 9 N3 row updated (complete; SHA recorded at N4
kickoff).

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

src/AcDream.Core.Net/Transport/TransportStats.cs

@@ -47,6 +47,11 @@ internal sealed class TransportStats
     /// pre-draw) plus one per checksum-failure re-park.</summary>
     public long KeysParked;
 
+    /// <summary>N3: cumulative <c>AckSequence</c> packets emitted by the
+    /// 2.0 s sweep (<c>SharedNet::EnqueuePak @ 0x00543B10</c> — retail's
+    /// only ack construction site; there is no per-packet ack).</summary>
+    public long AcksSent;
+
     /// <summary>Live sent-packet cache depth — the N5 watchdog value
     /// (<c>cache=N</c> in <c>[net-tick]</c>; the cache is unbounded like
     /// retail's, so depth is the health signal, not a cap).</summary>