feat(ios): device client-cert mTLS — ClientTLS package, transport wiring, install UX (C-iOS)

- ios/Packages/ClientTLS: SecIdentity wrapper, PKCS12 importer (typed errors), keychain store
  (AfterFirstUnlockThisDeviceOnly), pure MutualTLSChallengeResponder truth table, cross-platform
  X.509 DER summary. 14/14 tests.
- Both transports (SessionCore URLSessionTermTransport, App URLSessionHTTPTransport) + SessionThumbnail
  take a lazy @Sendable ()->ClientIdentity? provider: WS resolves per-connect, HTTP per client-cert
  challenge, so a freshly-imported cert applies without an app relaunch. AppEnvironment injects
  { store.loadedIdentityOrNil() }.
- ClientCertScreen (.fileImporter([.pkcs12]) + passphrase -> import -> keychain), reachable via a
  设备证书 entry in SessionListScreen.hostMenu. PairingViewModel gates tunnel-host probes on cert
  presence and re-maps mTLS-reject to a clientCertRejected message.
Verified: ClientTLS 14/14, SessionCore 93/93, xcodegen + xcodebuild BUILD SUCCEEDED.

ios/Packages/ClientTLS/Tests/ClientTLSTests/MutualTLSChallengeResponderTests.swift

@@ -0,0 +1,114 @@
+import Foundation
+import Testing
+@testable import ClientTLS
+
+// C-iOS-1 · The responder truth table: 3 challenge types × identity present /
+// absent. Exercised through the pure method-keyed core AND through a real
+// `URLAuthenticationChallenge` (proving the method is extracted correctly),
+// with no live socket.
+
+private let responder = MutualTLSChallengeResponder()
+
+private func makeIdentity() throws -> ClientIdentity {
+    try PKCS12Importer.importIdentity(
+        data: ClientTLSFixtures.deviceP12Data, passphrase: ClientTLSFixtures.passphrase
+    )
+}
+
+// MARK: - ClientCertificate
+
+@Test("ClientCertificate + identity → .useCredential with a credential")
+func clientCertWithIdentityUsesCredential() throws {
+    // Arrange
+    let identity = try makeIdentity()
+
+    // Act
+    let resolution = responder.resolve(
+        authenticationMethod: NSURLAuthenticationMethodClientCertificate, identity: identity
+    )
+
+    // Assert
+    #expect(resolution.disposition == .useCredential)
+    #expect(resolution.credential != nil)
+    #expect(resolution.credential?.identity != nil)
+}
+
+@Test("ClientCertificate + no identity → .cancelAuthenticationChallenge, no credential")
+func clientCertWithoutIdentityCancels() {
+    // Act
+    let resolution = responder.resolve(
+        authenticationMethod: NSURLAuthenticationMethodClientCertificate, identity: nil
+    )
+
+    // Assert
+    #expect(resolution.disposition == .cancelAuthenticationChallenge)
+    #expect(resolution.credential == nil)
+}
+
+// MARK: - ServerTrust
+
+@Test("ServerTrust → .performDefaultHandling regardless of identity")
+func serverTrustDefaultHandling() throws {
+    let identity = try makeIdentity()
+    for id in [identity, nil] as [ClientIdentity?] {
+        let resolution = responder.resolve(
+            authenticationMethod: NSURLAuthenticationMethodServerTrust, identity: id
+        )
+        #expect(resolution.disposition == .performDefaultHandling)
+        #expect(resolution.credential == nil)
+    }
+}
+
+// MARK: - Other method (e.g. Basic)
+
+@Test("an unrelated method → .performDefaultHandling regardless of identity")
+func otherMethodDefaultHandling() throws {
+    let identity = try makeIdentity()
+    for id in [identity, nil] as [ClientIdentity?] {
+        let resolution = responder.resolve(
+            authenticationMethod: NSURLAuthenticationMethodHTTPBasic, identity: id
+        )
+        #expect(resolution.disposition == .performDefaultHandling)
+        #expect(resolution.credential == nil)
+    }
+}
+
+// MARK: - Full URLAuthenticationChallenge extraction
+
+@Test("resolve(challenge:) extracts the method from the protection space")
+func resolveFromRealChallenge() throws {
+    // Arrange — a real challenge for the ClientCertificate method.
+    let identity = try makeIdentity()
+    let challenge = makeChallenge(method: NSURLAuthenticationMethodClientCertificate)
+
+    // Act
+    let withIdentity = responder.resolve(challenge, identity: identity)
+    let withoutIdentity = responder.resolve(challenge, identity: nil)
+
+    // Assert
+    #expect(withIdentity.disposition == .useCredential)
+    #expect(withIdentity.credential != nil)
+    #expect(withoutIdentity.disposition == .cancelAuthenticationChallenge)
+}
+
+// MARK: - Helpers
+
+/// Minimal sender so `URLAuthenticationChallenge` can be constructed in-process
+/// (its designated init requires a non-optional sender). The responder never
+/// calls back into the sender — it only reads `protectionSpace`.
+private final class NoopChallengeSender: NSObject, URLAuthenticationChallengeSender {
+    func use(_ credential: URLCredential, for challenge: URLAuthenticationChallenge) {}
+    func continueWithoutCredential(for challenge: URLAuthenticationChallenge) {}
+    func cancel(_ challenge: URLAuthenticationChallenge) {}
+}
+
+private func makeChallenge(method: String) -> URLAuthenticationChallenge {
+    let space = URLProtectionSpace(
+        host: "t1.terminal.yaojia.wang", port: 443, protocol: "https",
+        realm: nil, authenticationMethod: method
+    )
+    return URLAuthenticationChallenge(
+        protectionSpace: space, proposedCredential: nil, previousFailureCount: 0,
+        failureResponse: nil, error: nil, sender: NoopChallengeSender()
+    )
+}