feat: add authenticators, Account, and ClientPermissions (Tasks 3-7, 9)

- Account: per-account SubList and client tracking
- IAuthenticator interface, AuthResult, ClientAuthContext
- TokenAuthenticator: constant-time token comparison
- UserPasswordAuthenticator: multi-user with bcrypt/plain support
- SimpleUserPasswordAuthenticator: single user/pass config
- NKeyAuthenticator: Ed25519 nonce signature verification
- ClientPermissions: SubList-based publish/subscribe authorization

src/NATS.Server/Auth/SimpleUserPasswordAuthenticator.cs

@@ -0,0 +1,61 @@
+using System.Security.Cryptography;
+using System.Text;
+
+namespace NATS.Server.Auth;
+
+/// <summary>
+/// Authenticates a single username/password pair configured on the server.
+/// Supports plain-text and bcrypt-hashed passwords.
+/// Uses constant-time comparison for both username and password to prevent timing attacks.
+/// Reference: golang/nats-server/server/auth.go checkClientAuth for single user.
+/// </summary>
+public sealed class SimpleUserPasswordAuthenticator : IAuthenticator
+{
+    private readonly byte[] _expectedUsername;
+    private readonly string _serverPassword;
+
+    public SimpleUserPasswordAuthenticator(string username, string password)
+    {
+        _expectedUsername = Encoding.UTF8.GetBytes(username);
+        _serverPassword = password;
+    }
+
+    public AuthResult? Authenticate(ClientAuthContext context)
+    {
+        var clientUsername = context.Opts.Username;
+        if (string.IsNullOrEmpty(clientUsername))
+            return null;
+
+        var clientUsernameBytes = Encoding.UTF8.GetBytes(clientUsername);
+        if (!CryptographicOperations.FixedTimeEquals(clientUsernameBytes, _expectedUsername))
+            return null;
+
+        var clientPassword = context.Opts.Password ?? string.Empty;
+
+        if (!ComparePasswords(_serverPassword, clientPassword))
+            return null;
+
+        return new AuthResult { Identity = clientUsername };
+    }
+
+    private static bool ComparePasswords(string serverPassword, string clientPassword)
+    {
+        // Bcrypt hashes start with "$2" (e.g., $2a$, $2b$, $2y$)
+        if (serverPassword.StartsWith("$2"))
+        {
+            try
+            {
+                return BCrypt.Net.BCrypt.Verify(clientPassword, serverPassword);
+            }
+            catch
+            {
+                return false;
+            }
+        }
+
+        // Plain-text: constant-time comparison to prevent timing attacks
+        var serverBytes = Encoding.UTF8.GetBytes(serverPassword);
+        var clientBytes = Encoding.UTF8.GetBytes(clientPassword);
+        return CryptographicOperations.FixedTimeEquals(serverBytes, clientBytes);
+    }
+}