fix: address code quality review findings for batch 1

- SubjectsCollide: split tokens once upfront instead of O(n²) TokenAt calls
- NatsHeaderParser: manual digit accumulation avoids string allocation and overflow
- NatsHeaders: use IReadOnlyDictionary for Headers, immutable Invalid sentinel
- PermissionLruCache: add missing Count property

src/NATS.Server/Auth/PermissionLruCache.cs

@@ -35,6 +35,17 @@ public sealed class PermissionLruCache
         }
     }
 
+    public int Count
+    {
+        get
+        {
+            lock (_lock)
+            {
+                return _map.Count;
+            }
+        }
+    }
+
     public void Set(string key, bool value)
     {
         lock (_lock)