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/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++;