feat: M4a host key 校验（TOFU pin，关闭 MITM 敞口）

- TXCore/SSH: SSHWire(string/mpint 编码)、ECDSAConv(DER→SSH 签名/P256 blob)、
  HostKey(opensshFingerprint/evaluate/HostTriple/KnownHostsStore)，+10 单测（含真实指纹向量、高位 DER）
- TXTransport: SSHConfig.hostKeyVerifier 闭包 + SSHError.hostKeyMismatch；
  SSHSession 握手后 libssh2_session_hostkey 取 blob 交 verifier，拒绝则断开抛错
- app: 文件后端 KnownHostsStore（未签名 app 无 keychain 权限 SecItem -34018，host key 是
  公钥非机密，沙盒文件对 TOFU 足够）+ TOFU verifier + firstUse 横幅 + mismatch alert(信任/取消)
- 验证(192.168.9.199)：首次信任 pin+接受连上；假 pin→不符→断开+告警，本次指纹与 ssh-keygen 逐字符一致

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

apps/TerminalX/iOS/KeychainKnownHosts.swift

@@ -0,0 +1,54 @@
+import Foundation
+import TXCore
+
+/// known_hosts pin 存储（文件后端）。
+///
+/// 为何用文件而非 Keychain：本 app 当前未签名（CODE_SIGNING_ALLOWED=NO），无 keychain-access-group
+/// 权限 → SecItem 全返 errSecMissingEntitlement(-34018)。而 host key pin 是**公钥完整性数据、非机密**，
+/// 存于 app 沙盒文件对 TOFU 足够（威胁模型是 MITM 换 key，沙盒文件完整性对非越狱设备与 Keychain 相当）。
+/// 生产签名构建可换 Keychain 加固（另留 KnownHostsStore 协议便于替换）。
+///
+/// 格式：JSON `{ "egress|host|port": { "blob": "<base64>", "type": <int> } }`，
+/// 落 Application Support/known-hosts.json。
+final class KeychainKnownHostsStore: KnownHostsStore, @unchecked Sendable {
+    private let lock = NSLock()
+    private let url: URL
+    private var cache: [String: [String: Any]]
+
+    init() {
+        let base = NSSearchPathForDirectoriesInDomains(.applicationSupportDirectory, .userDomainMask, true)[0]
+        try? FileManager.default.createDirectory(atPath: base, withIntermediateDirectories: true)
+        url = URL(fileURLWithPath: base).appendingPathComponent("known-hosts.json")
+        if let data = try? Data(contentsOf: url),
+           let obj = try? JSONSerialization.jsonObject(with: data) as? [String: [String: Any]] {
+            cache = obj
+        } else {
+            cache = [:]
+        }
+    }
+
+    func lookup(_ triple: HostTriple) -> HostKeyRecord? {
+        lock.lock(); defer { lock.unlock() }
+        guard let entry = cache[triple.accountKey],
+              let b64 = entry["blob"] as? String, let blob = Data(base64Encoded: b64),
+              let type = entry["type"] as? Int else { return nil }
+        return HostKeyRecord(blob: blob, keyType: Int32(type))
+    }
+
+    func pin(_ triple: HostTriple, record: HostKeyRecord) {
+        lock.lock(); defer { lock.unlock() }
+        cache[triple.accountKey] = ["blob": record.blob.base64EncodedString(), "type": Int(record.keyType)]
+        persist()
+    }
+
+    func remove(_ triple: HostTriple) {
+        lock.lock(); defer { lock.unlock() }
+        cache[triple.accountKey] = nil
+        persist()
+    }
+
+    private func persist() {
+        guard let data = try? JSONSerialization.data(withJSONObject: cache) else { return }
+        try? data.write(to: url, options: .atomic)
+    }
+}