feat: M4b publickey 认证（SE 优先/软件回退，sign_callback）

- TXTransport: SSHSigner 协议 + Authentication.publicKeyCallback；SSHSession 用
  @convention(c) sshSignCallback(abstract 桥 SignerBox + malloc 出参)接 libssh2_userauth_publickey
- app SigningKeyProvider: SE 优先(kSecAttrTokenIDSecureEnclave)，未签名/无 SE 回退软件 P-256
  (导出落文件持久化)，同一 SSHSigner 路径；SecKeyCreateSignature(ecdsa)→ECDSAConv.derToSSHSignature
- 公钥 blob=ecdsa-sha2-nistp256||nistp256||X963；-txPubkeyAuth 日志打 authorized_keys 行
- 验证(192.168.9.199)：不给密码，公钥加 authorized_keys 后免密登录成功(whoami=kid)；
  模拟器 SE 生成 hw=true 可达，真机 SE 硬件签名+keychain 持久化待补

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/TerminalX/iOS/SigningKeyProvider.swift

@@ -0,0 +1,98 @@
+import Foundation
+import Security
+import TXCore
+import TXTransport
+
+/// SSH publickey 认证的签名器。私钥优先放 Secure Enclave（不可导出）；
+/// 模拟器/无 SE 环境回退软件 P-256（可导出，落沙盒文件持久化）——**同一份 SSHSigner 代码路径**，
+/// 差异仅生成参数一行，把真机验证压缩到"SE 生成/签名成功"两行。
+final class SigningKeyProvider: SSHSigner, @unchecked Sendable {
+    private let privateKey: SecKey
+    let publicKeyBlob: [UInt8]
+    let isHardwareBacked: Bool
+
+    /// authorized_keys 行（复制到服务器即可用本 key 登录）。
+    var authorizedKeysLine: String {
+        "ecdsa-sha2-nistp256 " + Data(publicKeyBlob).base64EncodedString() + " terminalx@device"
+    }
+
+    private init(privateKey: SecKey, hardware: Bool) {
+        self.privateKey = privateKey
+        self.isHardwareBacked = hardware
+        let pub = SecKeyCopyPublicKey(privateKey)!
+        let x963 = SecKeyCopyExternalRepresentation(pub, nil)! as Data   // 0x04||X||Y (65B)
+        self.publicKeyBlob = ECDSAConv.p256PublicKeyBlob(x963: [UInt8](x963))
+    }
+
+    // MARK: - SSHSigner
+
+    func signSSH(_ data: [UInt8]) -> [UInt8]? {
+        var err: Unmanaged<CFError>?
+        guard let der = SecKeyCreateSignature(
+            privateKey, .ecdsaSignatureMessageX962SHA256, Data(data) as CFData, &err) as Data? else {
+            NSLog("M4DBG SecKeyCreateSignature failed: \(String(describing: err?.takeRetainedValue()))")
+            return nil
+        }
+        return ECDSAConv.derToSSHSignature([UInt8](der))   // DER SEQUENCE{r,s} → SSH mpint r||mpint s
+    }
+
+    // MARK: - 生成 / 加载
+
+    private static var keyFileURL: URL {
+        let base = NSSearchPathForDirectoriesInDomains(.applicationSupportDirectory, .userDomainMask, true)[0]
+        return URL(fileURLWithPath: base).appendingPathComponent("ssh-ecdsa-p256.key")
+    }
+
+    static func loadOrCreate() -> SigningKeyProvider? {
+        // 1) 软件密钥文件（模拟器持久化，跨启动稳定）。
+        if let data = try? Data(contentsOf: keyFileURL),
+           let key = SecKeyCreateWithData(data as CFData, [
+               kSecAttrKeyType: kSecAttrKeyTypeECSECPrimeRandom,
+               kSecAttrKeyClass: kSecAttrKeyClassPrivate,
+               kSecAttrKeySizeInBits: 256,
+           ] as CFDictionary, nil) {
+            return SigningKeyProvider(privateKey: key, hardware: false)
+        }
+        // 2) Secure Enclave（真机；不可导出）。
+        if let se = createSecureEnclaveKey() {
+            return SigningKeyProvider(privateKey: se, hardware: true)
+        }
+        // 3) 回退软件 P-256（模拟器/无 SE），导出落文件。
+        guard let sw = createSoftwareKey() else { return nil }
+        if let ext = SecKeyCopyExternalRepresentation(sw, nil) as Data? {
+            try? ext.write(to: keyFileURL, options: .atomic)
+        }
+        return SigningKeyProvider(privateKey: sw, hardware: false)
+    }
+
+    private static func createSecureEnclaveKey() -> SecKey? {
+        guard let access = SecAccessControlCreateWithFlags(
+            nil, kSecAttrAccessibleWhenUnlockedThisDeviceOnly, .privateKeyUsage, nil) else { return nil }
+        let attrs: [String: Any] = [
+            kSecAttrKeyType as String: kSecAttrKeyTypeECSECPrimeRandom,
+            kSecAttrKeySizeInBits as String: 256,
+            kSecAttrTokenID as String: kSecAttrTokenIDSecureEnclave,
+            kSecPrivateKeyAttrs as String: [
+                kSecAttrIsPermanent as String: true,      // 持久化到 keychain（真机签名构建）；未签名/模拟器会失败(-34018)→回退软件+文件
+                kSecAttrApplicationTag as String: "ai.athom.terminalx.sshkey".data(using: .utf8)!,
+                kSecAttrAccessControl as String: access,
+            ],
+        ]
+        var err: Unmanaged<CFError>?
+        let k = SecKeyCreateRandomKey(attrs as CFDictionary, &err)
+        if k == nil { NSLog("M4DBG SE key unavailable (预期于模拟器): \(String(describing: err?.takeRetainedValue()))") }
+        return k
+    }
+
+    private static func createSoftwareKey() -> SecKey? {
+        let attrs: [String: Any] = [
+            kSecAttrKeyType as String: kSecAttrKeyTypeECSECPrimeRandom,
+            kSecAttrKeySizeInBits as String: 256,
+            kSecPrivateKeyAttrs as String: [kSecAttrIsPermanent as String: false],
+        ]
+        var err: Unmanaged<CFError>?
+        let k = SecKeyCreateRandomKey(attrs as CFDictionary, &err)
+        if k == nil { NSLog("M4DBG software key create failed: \(String(describing: err?.takeRetainedValue()))") }
+        return k
+    }
+}