diag(net): #260 outbound/command-gate probe + corrected issue framing

The two-agent investigation refuted #260's as-filed hypotheses: every
UseWithTarget was acked (the J5.2 use gate never latched), and the LOH
leak is bounded sawtooth churn - the real climb is ~2.25 GB of native/
GPU memory (WS 3,261 vs managed 1,015 MiB at wedge). The wedge evidence
also showed why it could hide: the live combat toggle routes through the
generation-gated runtime command seam, and every rejection exit in that
chain (Disposed / StaleGeneration / !IsInWorld at Validate, plus the
operations slot reading IsInWorld=false when unbound) is COMPLETELY
silent - no log, no event.

ACDREAM_PROBE_NET=1 (NetDiagnostics owner, PhysicsDiagnostics pattern)
now arms three probe families, all zero-cost when off:

- [net-out] per reliable send at the SendGameMessage chokepoint: opcode,
  GameAction type+sequence, fragment/packet sequence, managed thread id
  (two tids would prove the cross-thread ISAAC-desync hypothesis alone),
  and state; [net-out-EX] via an exception FILTER that logs without
  catching, so propagation is unchanged.
- [net-tick] 1 Hz cadence from WorldSession.Tick: inbound/s, queue
  depth, budget breaks, worst inter-tick gap (frame-stall witness),
  out/s, acks/s.
- [cmd-gate] every silent runtime-command rejection with expected-vs-
  view generation, lifecycle, and IsInWorld, plus the combat toggle
  result (whose Inactive exit reads a DIFFERENT IsInWorld source).

One walked-portal repro session with this probe distinguishes all
remaining #260 wedge hypotheses. ISSUES.md #260 rewritten to the
corrected two-root framing.

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

src/AcDream.Core.Net/NetDiagnostics.cs

@@ -0,0 +1,36 @@
+namespace AcDream.Core.Net;
+
+/// <summary>
+/// Diagnostic owner for the <c>ACDREAM_PROBE_NET</c> probe family (#260).
+/// Read once at startup, following the <c>PhysicsDiagnostics</c> pattern.
+///
+/// <para>
+/// When enabled, three probe line families are emitted:
+/// <list type="bullet">
+/// <item><c>[net-out]</c> — one line per outbound reliable game message at the
+/// <c>WorldSession.SendGameMessage</c> chokepoint: opcode, game-action type +
+/// sequence (when the body is a 0xF7B1 GameAction), fragment/packet sequence,
+/// managed thread id, and session state. An exception escaping the wire write
+/// additionally emits <c>[net-out-EX]</c> via an exception filter (log
+/// without catching — behavior is unchanged).</item>
+/// <item><c>[net-tick]</c> — a once-per-second cadence summary from
+/// <c>WorldSession.Tick</c>: inbound datagrams/s, remaining queue depth,
+/// budget-break count, worst inter-tick gap (= worst frame stall as seen by
+/// the net pump), outbound sends/s, and acks/s.</item>
+/// <item><c>[cmd-gate]</c> — one line per generation-gated runtime command
+/// REJECTION in <c>CurrentGameRuntimeCommandAdapter.Validate</c> (status,
+/// expected vs view generation, lifecycle, IsInWorld) plus the combat-toggle
+/// result. These rejections are otherwise completely silent, which is what
+/// let #260's wedge hide.</item>
+/// </list>
+/// Zero steady-state cost when off: every site is behind this single bool.
+/// </para>
+/// </summary>
+public static class NetDiagnostics
+{
+    /// <summary>
+    /// <c>ACDREAM_PROBE_NET=1</c> — #260 outbound/command-gate probe family.
+    /// </summary>
+    public static bool ProbeNet { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_NET") == "1";
+}