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/NKeyAuthenticator.cs

@@ -0,0 +1,66 @@
+using NATS.NKeys;
+
+namespace NATS.Server.Auth;
+
+/// <summary>
+/// Authenticates clients using NKey (Ed25519) public-key signature verification.
+/// The server sends a random nonce in the INFO message. The client signs the nonce
+/// with their private key and sends the public key + base64-encoded signature in CONNECT.
+/// The server verifies the signature against the registered NKey users.
+/// </summary>
+/// <remarks>
+/// Reference: golang/nats-server/server/auth.go — checkNKeyAuth
+/// </remarks>
+public sealed class NKeyAuthenticator(IEnumerable<NKeyUser> nkeyUsers) : IAuthenticator
+{
+    private readonly Dictionary<string, NKeyUser> _nkeys = nkeyUsers.ToDictionary(
+        u => u.Nkey,
+        u => u,
+        StringComparer.Ordinal);
+
+    public AuthResult? Authenticate(ClientAuthContext context)
+    {
+        var clientNkey = context.Opts.Nkey;
+        if (string.IsNullOrEmpty(clientNkey))
+            return null;
+
+        if (!_nkeys.TryGetValue(clientNkey, out var nkeyUser))
+            return null;
+
+        var clientSig = context.Opts.Sig;
+        if (string.IsNullOrEmpty(clientSig))
+            return null;
+
+        try
+        {
+            // Decode base64 signature (handle both standard and URL-safe base64)
+            byte[] sigBytes;
+            try
+            {
+                sigBytes = Convert.FromBase64String(clientSig);
+            }
+            catch (FormatException)
+            {
+                // Try URL-safe base64 by converting to standard base64
+                var padded = clientSig.Replace('-', '+').Replace('_', '/');
+                padded = padded.PadRight(padded.Length + (4 - padded.Length % 4) % 4, '=');
+                sigBytes = Convert.FromBase64String(padded);
+            }
+
+            var kp = KeyPair.FromPublicKey(clientNkey);
+            if (!kp.Verify(context.Nonce, sigBytes))
+                return null;
+        }
+        catch
+        {
+            return null;
+        }
+
+        return new AuthResult
+        {
+            Identity = clientNkey,
+            AccountName = nkeyUser.Account,
+            Permissions = nkeyUser.Permissions,
+        };
+    }
+}