feat(admin): replace dead ThreadPool counters with a network readout (#2663)

## Summary

The admin gump's **Performance** page still rendered `ThreadPool` worker / IOCP quotas — a RunUO leftover from `Socket.BeginReceive` days. Both figures are dead: IORingGroup drives RIO / io_uring / epoll / kqueue synchronously on the game loop, so the IOCP figure reads 0% forever, and the real off-loop work (serialization workers, password checks, advanced search, console input) runs on dedicated threads the worker figure never sees.

This replaces the block with a read-only network readout.

## Engine

- `NetState.GetNetworkStats()` — new public accessor returning a `NetworkStats` snapshot (ModernUO-owned record types in `Network/NetworkStats.cs`, so UOContent takes no dependency on transport types):
  - live: connected / max sockets, authenticated count, per-tier growth usage, the three loop queues (throttled, flush pending, pending disconnects)
  - configured: base / pre-auth / max buffer sizes, growth budget, memory ceiling, last-read available memory
  - `LastSweep`: everything the once-a-minute maintenance sweep saw, including the refusal counters it resets
- The sweep already kept four of its results (`_lastTier*`, `_lastBaseCapacityBytes`) for change detection; those fold into one `SendBufferSweep` snapshot. Sweep logic and logging are unchanged.
- The accessor never calls `Maintain()`, so opening the page cannot rotate the retention window.
- Tick age is compared by subtraction; "has a sweep run" is a `bool`, not a zero-tick sentinel.

## Gump

```
Event Loop:    Healthy
Connections:   123 / 4096 (117 authenticated)
Loop Queues:   throttled 0, flush 3, closing 0
Send Buffers:  [scrollable]
  Base: 64.0 KB recv, 256.0 KB send; pre-auth 4.0 KB / 4.0 KB
  Base pools: 10.2 MB
  Growth tiers (live):
    512.0 KB: 3 / 32 in use, floor 5
    1.0 MB: 0 / 0 in use, floor 0
    2.0 MB: 0 / 0 in use, floor 0
  Budget: 16.0 MB of 256.0 MB, max 2.0 MB per socket
  Ceiling: 80% of 7.8 GB available
  Last sweep 42s ago: released 0 tier, 0 base
  Refused: budget 0, at max 0, ceiling 0
```

Projects/Server/Network/NetState/NetState.Network.cs

@@ -56,10 +56,7 @@ public partial class NetState
     internal static long _availableMemoryBytes;
 
     private static Timer.DelayCallTimer _maintenanceTimer;
-    private static long _lastTierCapacityBytes;
-    private static int _lastTierInUse;
-    private static int _lastTierRetainFloor;
-    private static long _lastBaseCapacityBytes;
+    private static SendBufferSweep _lastSweep;
 
     private static readonly Queue<NetState> _disposed = [];
     private static readonly TimeSpan ConnectingSocketIdleLimit = TimeSpan.FromMilliseconds(5000); // 5 seconds
@@ -238,14 +235,23 @@ public partial class NetState
         _capRefusals = 0;
 
         var stats = _socketManager.Maintain();
-        var changed = stats.TierCapacityBytes != _lastTierCapacityBytes ||
-                      stats.TierInUse != _lastTierInUse ||
-                      stats.TierRetainFloor != _lastTierRetainFloor ||
-                      stats.BaseCapacityBytes != _lastBaseCapacityBytes;
-        _lastTierCapacityBytes = stats.TierCapacityBytes;
-        _lastTierInUse = stats.TierInUse;
-        _lastTierRetainFloor = stats.TierRetainFloor;
-        _lastBaseCapacityBytes = stats.BaseCapacityBytes;
+        var changed = stats.TierCapacityBytes != _lastSweep.TierCapacityBytes ||
+                      stats.TierInUse != _lastSweep.TierInUse ||
+                      stats.TierRetainFloor != _lastSweep.TierRetainFloor ||
+                      stats.BaseCapacityBytes != _lastSweep.BaseCapacityBytes;
+        _lastSweep = new SendBufferSweep(
+            true,
+            Core.TickCount,
+            stats.TierCapacityBytes,
+            stats.TierInUse,
+            stats.TierRetainFloor,
+            stats.BuffersReleased,
+            stats.BaseCapacityBytes,
+            stats.BaseBuffersReleased,
+            stats.GrowthRefusals,
+            capRefusals,
+            ceilingRefusals
+        );
 
         // Quiet unless something moved
         if (changed || stats.BuffersReleased > 0 || stats.BaseBuffersReleased > 0 || stats.GrowthRefusals > 0 ||
@@ -266,6 +272,48 @@ public partial class NetState
         }
     }
 
+    /// <summary>
+    /// Read-only diagnostics snapshot; never rotates the maintenance window. Cold path: allocates.
+    /// </summary>
+    public static NetworkStats GetNetworkStats()
+    {
+        var manager = _socketManager;
+        var tiers = manager == null ? [] : new SendBufferTierUsage[manager.SendBufferTierCount];
+        for (var i = 0; i < tiers.Length; i++)
+        {
+            var tier = manager.GetSendBufferTierStats(i);
+            tiers[i] = new SendBufferTierUsage(tier.BufferSize, tier.Capacity, tier.InUse, tier.RetainFloor);
+        }
+
+        var authenticated = 0;
+        foreach (var ns in _instances)
+        {
+            if (ns.Account != null)
+            {
+                authenticated++;
+            }
+        }
+
+        return new NetworkStats(
+            manager?.ConnectedCount ?? 0,
+            manager?.MaxSockets ?? MaxConnections,
+            authenticated,
+            RecvBufferSize,
+            SendBufferSize,
+            MaxSendBufferSize,
+            InitialRecvBufferSize,
+            InitialSendBufferSize,
+            _sendBufferGrowthBudget,
+            _memoryCeilingPercent,
+            _availableMemoryBytes,
+            tiers,
+            _lastSweep,
+            _throttled.Count + _throttledPending.Count,
+            _flushPending.Count,
+            _pendingDisconnects.Count
+        );
+    }
+
     /// <summary>
     /// Slabs each base pool is divided into, after the transport applies its minimum slab size.
     /// </summary>