refactor(battle-node): gate WS diagnostic logging behind config flag

The temporary [sio-in] / [sio-out] / [ws-rx-text] / [ws-rx-bin] /
[ws-recv-exit] / [ws-loop-exit] logs added during the hand-ack
investigation are useful enough to keep around (PvP testing, future WS
debugging) but too chatty to leave on by default. Promote them from
"strip before merge" to a permanent opt-in.

New BattleNodeOptions.DiagnosticLogging (bool, default false). Wired
through BattleNodeWebSocketHandler to RealParticipant via a new optional
ctor parameter (default false — existing test sites pick up the silent
default with no changes). Every Information/Warning log added during the
investigation is now if-gated; non-diagnostic logs (the decode-failure
warnings, the dispatch-drop debug) stay as-is.

Toggle via appsettings*.json:
  "BattleNode": { "DiagnosticLogging": true }

Or live via the singleton:
  factory.Services.GetRequiredService<BattleNodeOptions>().DiagnosticLogging = true

175 battle-node tests still passing — existing tests use the constructor
default and emit nothing, so no test changes were required.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.BattleNode/Bridge/BattleNodeOptions.cs

@@ -26,4 +26,18 @@ public sealed class BattleNodeOptions
     /// is the only way to get PvP behavior.</para>
     /// </summary>
     public bool SoloDefaultsToScripted { get; set; } = false;
+
+    /// <summary>
+    /// When true, <see cref="Sessions.Participants.RealParticipant"/> emits per-frame
+    /// diagnostic logs at Information level: <c>[sio-in]</c> on every inbound msg/alive/hand
+    /// envelope (URI, pubSeq, ackId, dispatch decision, ack-sent flag, ack arg, inbound
+    /// watermark); <c>[sio-out]</c> on every outbound push (URI, pubSeq, playSeq, noStock);
+    /// <c>[ws-rx-text]</c> / <c>[ws-rx-bin]</c> on every WS frame received at the transport
+    /// layer; <c>[ws-recv-exit]</c> / <c>[ws-loop-exit]</c> on read-loop termination
+    /// (with WebSocket state + exception type when applicable). Default false — keeps
+    /// production logs clean. Flip on per session for live WS debugging, PvP investigation,
+    /// or to reproduce the kind of softlock chased in
+    /// <c>docs/audits/battle-node-sio-events-2026-06-02.md</c>.
+    /// </summary>
+    public bool DiagnosticLogging { get; set; } = false;
 }