feat!: genesis-pinned operator set, server identity with an encrypted control link, key-less join codes; release key continuity

Join codes need no passkey touch and carry only public values (genesis
fingerprint, owner credentials, installer hashes). Nodes pin the genesis and
owners at enrollment and only follow passkey-signed operator set versions.
The panel's server key is listed in the set; pinned nodes require its
signature and run the control link under AES-256-GCM. Password sessions carry
no capabilities. The updater adopts a next_key from a signed manifest.

Bump version to 0.2.0. [CI SKIP]

backend/fleet/src/Fleet.Identity/SessionKeys.cs

@@ -16,6 +16,14 @@ public sealed class EphemeralKey : IDisposable
 
     public byte[] PublicKeySpki => _key.ExportSubjectPublicKeyInfo();
 
+    /// <summary>与对端的临时公钥做 ECDH，返回原始共享秘密；必须再经 KDF 才能当密钥用。</summary>
+    public byte[] AgreeRaw(ReadOnlySpan<byte> remotePublicKeySpki)
+    {
+        using ECDiffieHellman remote = ECDiffieHellman.Create();
+        remote.ImportSubjectPublicKeyInfo(remotePublicKeySpki, out _);
+        return _key.DeriveRawSecretAgreement(remote.PublicKey);
+    }
+
     /// <summary>
     /// 与对端的临时公钥协商，再经 HKDF-SHA256 派生会话凭据。
     /// 盐是服务端的随机数，info 绑定双方的临时公钥，防止凭据被挪用到别的握手上。