fix(driver-galaxy): resolve Medium code-review finding (Driver.Galaxy-011)

GetMemoryFootprint() returned a constant 0 with a stale "PR 4.4 sets this" comment
even though PR 4.4 shipped the SubscriptionRegistry. Replace with a live estimate:
64 bytes × TrackedItemHandleCount + 256 bytes × TrackedSubscriptionCount. A 50k-tag
set now registers ~3 MB with the server's cache-flush heuristic instead of being
invisible. Returns 0 when no subscriptions are active.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs

@@ -499,7 +499,22 @@ public sealed class GalaxyDriver
     public IReadOnlyList<HostConnectivityStatus> GetHostStatuses() => _hostStatuses.Snapshot();
 
     /// <inheritdoc />
-    public long GetMemoryFootprint() => 0; // PR 4.4 sets this from SubscriptionRegistry size.
+    /// <remarks>
+    ///     Estimated footprint: 64 bytes × tracked item handles (one gw subscription entry
+    ///     per bound tag) + 256 bytes × tracked driver subscriptions (registry overhead per
+    ///     OPC UA monitored item). Returns 0 when no subscriptions are active. These
+    ///     constants are conservative — a 50k-tag set occupies ~3 MB and registers clearly
+    ///     with the server's cache-flush heuristic. Driver.Galaxy-011: the stale
+    ///     "PR 4.4 sets this" comment is removed; PR 4.4 shipped the SubscriptionRegistry
+    ///     but never wired it here.
+    /// </remarks>
+    public long GetMemoryFootprint()
+    {
+        const long BytesPerItemHandle = 64L;     // TagBinding + reverse-map entry
+        const long BytesPerSubscription = 256L;  // SubscriptionEntry overhead
+        return (_subscriptions.TrackedItemHandleCount * BytesPerItemHandle)
+             + (_subscriptions.TrackedSubscriptionCount * BytesPerSubscription);
+    }
 
     /// <inheritdoc />
     public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;