fix(driver-twincat): resolve Medium code-review finding (Driver.TwinCAT-012)

GetMemoryFootprint now returns tagsByName * 256 + nativeSubs * 512 bytes
instead of a hard-coded 0; document that the stream-and-discard symbol
browse leaves no flushable cache so FlushOptionalCachesAsync is a
deliberate no-op.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs

@@ -134,7 +134,22 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
     }
 
     public DriverHealth GetHealth() => _health;
-    public long GetMemoryFootprint() => 0;
+
+    /// <summary>
+    ///     Estimated bytes attributable to this driver instance (Driver.TwinCAT-012).
+    ///     This driver holds no flushable symbol cache — <c>BrowseSymbolsAsync</c> streams and
+    ///     discards; the footprint reflects live allocations only:
+    ///     ~256 bytes per pre-declared tag (tag-definition record + dictionary overhead) and
+    ///     ~512 bytes per active native subscription.
+    /// </summary>
+    public long GetMemoryFootprint() =>
+        (_tagsByName.Count * 256L) + (_nativeSubs.Count * 512L);
+
+    /// <summary>
+    ///     No flushable cache exists in this driver — the symbol table is streamed fresh on
+    ///     every <see cref="DiscoverAsync"/> call. This is a no-op but is deliberately present
+    ///     so Core's cache-budget enforcement sees a compliant Tier-A driver.
+    /// </summary>
     public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;
 
     internal int DeviceCount => _devices.Count;