fix(driver-galaxy): resolve Low code-review findings (Driver.Galaxy-005,010,012,013)

- Driver.Galaxy-005: rewrite the EventPump BoundedChannelOptions comment
  to honestly describe the Wait+TryWrite pattern.
- Driver.Galaxy-010: ResolveApiKey now warns when a literal API key is
  used in production wiring; added an explicit dev: prefix for known
  cleartext-in-dev cases and rewrote the GalaxyGatewayOptions doc.
- Driver.Galaxy-012: O(1) reverse-lookup for SubscriptionRegistry
  dispatch via per-entry FullRefByItemHandle map; immutable hash-set for
  the cross-binding reverse map; SubscribeAsync / ReadViaSubscribeOnce
  use BuildResultIndex for per-reference correlation.
- Driver.Galaxy-013: ReinitializeAsync now validates the incoming JSON
  against the running options; ReplayOnSessionLost honoured by the
  Replay path; class summary rewritten to describe the shipped surface.

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

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/SubscriptionRegistryTests.cs

@@ -183,6 +183,36 @@ public sealed class SubscriptionRegistryTests
         registry.ResolveSubscribers(0).ShouldBeEmpty();
     }
 
+    // ===== Driver.Galaxy-012 regression: ResolveSubscribers is O(1) per binding =====
+
+    [Fact]
+    public void ResolveSubscribers_LargeBindingSet_DispatchesCorrectly()
+    {
+        // 5000-tag subscription. ResolveSubscribers must still return the right
+        // full-reference for any item handle without a linear scan of the entire
+        // binding list — the old FirstOrDefault(b => b.ItemHandle == h) was O(n)
+        // per dispatch, so 50k tags × 1Hz fan-out was 50k linear scans per second.
+        var registry = new SubscriptionRegistryAccess();
+        const int tagCount = 5000;
+        var bindings = new List<TagBindingAccess>(tagCount);
+        for (var i = 0; i < tagCount; i++)
+        {
+            bindings.Add(new TagBindingAccess($"Tag.{i}", 1000 + i));
+        }
+        registry.Register(1, bindings);
+
+        // Pull the last entry — the worst case for a linear scan.
+        var subs = registry.ResolveSubscribers(1000 + tagCount - 1);
+
+        subs.Count.ShouldBe(1);
+        subs[0].FullReference.ShouldBe($"Tag.{tagCount - 1}");
+
+        // Mid-range entry too — proves the index isn't position-dependent.
+        var mid = registry.ResolveSubscribers(1000 + tagCount / 2);
+        mid.Count.ShouldBe(1);
+        mid[0].FullReference.ShouldBe($"Tag.{tagCount / 2}");
+    }
+
     // Internal types are accessed via friend assembly (InternalsVisibleTo); these
     // wrapper aliases keep the test code readable.
     private sealed class SubscriptionRegistryAccess