test(net): N0 - ACE-behaviour double, virtual clock, lossy link

Campaign N slice N0 (docs/plans/2026-07-29-network-transport-campaign.md):
the referee that slices N1-N5 are graded against, test-project only, zero
production changes.

- VirtualClock: Stopwatch-shaped deterministic time source (fixed 100 ns
  ticks) that N1 will inject behind the production TransportClock.
- AceCryptoModel: verbatim port of ACE CryptoSystem Search/ConsumeKey over
  our IsaacRandom - 256-key window, parked-key set, Headroom/OrphanCount
  diagnostics (CryptoSystem.cs:8-49 cited per method).
- AceSessionModel: transport-free ACE NetworkSession over raw datagrams,
  every rule cited to NetworkSession.cs - CRC-before-everything silent
  drop, cleartext-NAK early return (no timeout refresh, :283-308),
  60 s timeout refresh (:329-331), exact-equality ack dedup exemption
  (:342-347), desired+2 NAK trigger with 1 s limit (:351-363), >window
  AbnormalSequenceReceived (:393-397), the :474-476 watermark hole,
  ack-value cache prune (:663-673), fragment gate (:532-543), seq>=2
  caching (:730), Retransmission-flag resends with the ORIGINAL IssacXor
  (:675-686), RejectRetransmit, 2 s cleartext cumulative ack, 20 s
  TimeSync, EchoResponse, 120 s cache prune (:251-262). ACE's raw
  wrap-unsafe comparisons are modeled bug-for-bug, not fixed.
- LossyLink: deterministic drop/reorder/seeded-loss fault injector, pure
  data structure.
- FakeAceTransport: IWorldSessionTransport binding a REAL WorldSession to
  the model through the link, with the handshake scripted (ConnectRequest
  reusing the negotiation fixture layout, CharacterList, ServerReady,
  logoff confirmation) - genuine Connect/EnterWorld/Tick/Dispose with no
  sockets.
- 19 new tests pin the double, including
  CleartextNonAckAdvancesWatermark_TheAceHole (the self-induced wedge
  behind scope rows TS-57/TS-58/AP-125), re-key = permanent orphan,
  unrequested-resend window burn, the 115-id NAK cap boundary, and a
  full no-socket session lifecycle with both ISAAC streams verified
  aligned end-to-end.

Core.Net suite: 678 passed / 0 failed (659 existing + 19 new).

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

tests/AcDream.Core.Net.Tests/Transport/VirtualClock.cs

@@ -0,0 +1,54 @@
+namespace AcDream.Core.Net.Tests.Transport;
+
+/// <summary>
+/// Deterministic monotonic time source for transport tests. Exposes the same
+/// (timestamp, frequency) shape as <c>System.Diagnostics.Stopwatch</c>
+/// (<c>GetTimestamp()</c> + <c>Frequency</c>) so slice N1 can inject it
+/// behind the production <c>TransportClock</c> without adapting call sites.
+/// Time only moves when a test calls <see cref="Advance"/>.
+///
+/// <para>
+/// Deliberately dependency-free (System only) and NOT tied to the machine's
+/// <c>Stopwatch.Frequency</c>: a fixed 100 ns tick makes every gate
+/// computation reproducible across platforms.
+/// </para>
+/// </summary>
+internal sealed class VirtualClock
+{
+    /// <summary>
+    /// Fixed tick rate: 100 ns ticks (10,000,000 per second), equal to
+    /// <see cref="TimeSpan.TicksPerSecond"/> so <see cref="TimeSpan"/>
+    /// arithmetic maps 1:1 onto clock ticks.
+    /// </summary>
+    public const long TicksPerSecond = TimeSpan.TicksPerSecond;
+
+    private long _timestamp;
+
+    public VirtualClock(long startTimestamp = 0) => _timestamp = startTimestamp;
+
+    /// <summary><c>Stopwatch.Frequency</c> equivalent.</summary>
+    public long Frequency => TicksPerSecond;
+
+    /// <summary><c>Stopwatch.GetTimestamp()</c> equivalent.</summary>
+    public long GetTimestamp() => _timestamp;
+
+    /// <summary>
+    /// Seconds since the clock's epoch as a double — the shape of the
+    /// retail/ACE PortalYearTicks-style wall values written into TimeSync
+    /// payloads and the packet header's 16-bit <c>Time</c> field.
+    /// </summary>
+    public double Seconds => (double)_timestamp / TicksPerSecond;
+
+    /// <summary>Move time forward. The clock is monotonic — negative deltas throw.</summary>
+    public void Advance(TimeSpan delta)
+    {
+        if (delta < TimeSpan.Zero)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(delta),
+                "the clock is monotonic — it cannot go backwards");
+        }
+
+        _timestamp += delta.Ticks;
+    }
+}