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.Server.Abstractions/NodeRecords.cs

@@ -34,8 +34,7 @@ public sealed record NewNode(
     long NowMs,
     string Kind = "",
     byte[]? TlsPublicKey = null,
-    string? AddedBy = null,
-    MemberProofDto? Proof = null);
+    string? AddedBy = null);
 
 public sealed record JoinTokenRecord(
     string Id,
@@ -49,7 +48,6 @@ public sealed record JoinTokenRecord(
     long? RevokedAtMs,
     long CreatedAtMs,
     string Kind = "",
-    string? CreatedBy = null,
-    SignedStatementDto? Grant = null);
+    string? CreatedBy = null);
 
 public sealed record Presence(bool Online, long AtMs, string? NodeVersion, IReadOnlyList<string> Capabilities, string? HelloMetaJson);

backend/fleet/src/Fleet.Server.Abstractions/ServerContracts.cs

@@ -6,6 +6,18 @@ namespace Fleet.Server;
 
 public sealed record EnrollmentContext(string? RemoteIp, JsonElement? ProductMeta);
 
+/// <summary>
+/// 服务器的身份密钥：操作员用 passkey 把它的公钥签进操作员集合。有它的时候，认证挑战会带上服务器的签名，节点据此确认对面是真的服务器，
+/// 并在这条链路上用应用层加密。没有注册就是旧的行为（没有服务器身份）。
+/// </summary>
+public interface IServerIdentity
+{
+    byte[] PublicKeySpki { get; }
+
+    /// <summary>对一段内容签名（<c>Fleet.Identity.ServerHello.Payload</c> 给出的）。</summary>
+    byte[] Sign(ReadOnlySpan<byte> payload);
+}
+
 /// <summary>操作员集合的链（passkey 签的，创世在前）。没有初始化信任根时是空的。节点注册时用它确认「这个面板确实是我的操作员的」。</summary>
 public interface IOperatorTrustSource
 {