Fix GuestDataPool lease leak on non-GPU compute dispatch paths (#759)

ObserveComputeDispatch only returned evaluation's pooled arrays when
evaluationHandledByCpu was set. Dispatches rejected before submission
(empty resource tables, oversized workgroup, compile failure) or a
compute submit that got dropped instead of enqueued (workSequence == 0)
never handed those buffers to a consumer that would return them,
leaking one lease per occurrence and growing GuestDataPool.Shared
without bound over a long session.

Adds GuestDataPool.DiagnosticStats() (outstanding lease count, idle
cached bytes) surfaced in the periodic [LOADER][PERF] line, to catch
this class of regression going forward.

Verified on Ghost of Yotei and Demon's Souls: pool_leases grew
unbounded before the fix (525 in 5 min on Demon's Souls, 1114 in 180s
on Yotei) and stays flat/bounded after (0-6 and 2-18 respectively),
with no behavioral regression observed.

src/SharpEmu.Libs/Gpu/GuestDataPool.cs

@@ -23,6 +23,10 @@ internal static class GuestDataPool
 
     public static void Trim() => ((BoundedByteArrayPool)Shared).Trim();
 
+    /// <summary>Outstanding lease count and idle cached bytes, for leak diagnostics.</summary>
+    public static (int LeaseCount, ulong CachedBytes) DiagnosticStats() =>
+        ((BoundedByteArrayPool)Shared).Stats();
+
     private sealed class BoundedByteArrayPool : ArrayPool<byte>
     {
         private readonly object _gate = new();
@@ -119,6 +123,14 @@ internal static class GuestDataPool
             }
         }
 
+        public (int LeaseCount, ulong CachedBytes) Stats()
+        {
+            lock (_gate)
+            {
+                return (_leases.Count, _cachedBytes);
+            }
+        }
+
         private int GetAllocationLength(int minimumLength)
         {
             if (minimumLength <= 16)