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)

src/NATS.Server/Protocol/NatsHeaderParser.cs

@@ -1,20 +1,26 @@
+using System.Collections.ObjectModel;
 using System.Text;
 
 namespace NATS.Server.Protocol;
 
-public readonly struct NatsHeaders
+public readonly struct NatsHeaders()
 {
     public int Status { get; init; }
-    public string Description { get; init; }
-    public Dictionary<string, string[]> Headers { get; init; }
+    public string Description { get; init; } = string.Empty;
+    public IReadOnlyDictionary<string, string[]> Headers { get; init; } = ReadOnlyDictionary<string, string[]>.Empty;
 
-    public static readonly NatsHeaders Invalid = new() { Status = -1, Description = string.Empty, Headers = new() };
+    public static readonly NatsHeaders Invalid = new()
+    {
+        Status = -1,
+        Description = string.Empty,
+        Headers = ReadOnlyDictionary<string, string[]>.Empty,
+    };
 }
 
 public static class NatsHeaderParser
 {
-    private static readonly byte[] CrLf = "\r\n"u8.ToArray();
-    private static readonly byte[] Prefix = "NATS/1.0"u8.ToArray();
+    private static ReadOnlySpan<byte> CrLf => "\r\n"u8;
+    private static ReadOnlySpan<byte> Prefix => "NATS/1.0"u8;
 
     public static NatsHeaders Parse(ReadOnlySpan<byte> data)
     {
@@ -46,9 +52,10 @@ public static class NatsHeaderParser
             while (si < statusLine.Length && statusLine[si] >= (byte)'0' && statusLine[si] <= (byte)'9')
                 si++;
 
-            if (si > numStart)
+            if (si > numStart && si - numStart <= 5) // max 5 digits to avoid overflow
             {
-                status = int.Parse(Encoding.ASCII.GetString(statusLine[numStart..si]));
+                for (int idx = numStart; idx < si; idx++)
+                    status = status * 10 + (statusLine[idx] - '0');
 
                 while (si < statusLine.Length && statusLine[si] == (byte)' ')
                     si++;

src/NATS.Server/Subscriptions/SubjectMatch.cs

@@ -172,11 +172,13 @@ public static class SubjectMatch
         if (lit2 && !lit1)
             return MatchLiteral(subj2, subj1);
 
-        // Both have wildcards
-        int n1 = NumTokens(subj1);
-        int n2 = NumTokens(subj2);
-        bool hasFwc1 = subj1.Contains('>');
-        bool hasFwc2 = subj2.Contains('>');
+        // Both have wildcards — split once to avoid O(n²) TokenAt calls
+        var tokens1 = subj1.Split(Sep);
+        var tokens2 = subj2.Split(Sep);
+        int n1 = tokens1.Length;
+        int n2 = tokens2.Length;
+        bool hasFwc1 = tokens1[^1] == ">";
+        bool hasFwc2 = tokens2[^1] == ">";
 
         if (!hasFwc1 && !hasFwc2 && n1 != n2)
             return false;
@@ -188,9 +190,7 @@ public static class SubjectMatch
         int stop = Math.Min(n1, n2);
         for (int i = 0; i < stop; i++)
         {
-            var t1 = TokenAt(subj1, i);
-            var t2 = TokenAt(subj2, i);
-            if (!TokensCanMatch(t1, t2))
+            if (!TokensCanMatch(tokens1[i], tokens2[i]))
                 return false;
         }
         return true;