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/Sources/ClientTLS/ClientIdentity.swift

@@ -0,0 +1,55 @@
+import Foundation
+import Security
+
+/// C-iOS-1 · An imported device client identity: the `SecIdentity` (leaf
+/// certificate + its private key) plus the issuer chain that accompanies it in
+/// the TLS handshake.
+///
+/// `@unchecked Sendable`: `SecIdentity` / `SecCertificate` are CoreFoundation
+/// handles that are immutable once imported and thread-safe to read; this value
+/// only ever holds finished imports and never mutates them. Marking it lets the
+/// identity flow into the `@unchecked Sendable` WS connection and the URLSession
+/// delegates that answer client-certificate challenges.
+public struct ClientIdentity: @unchecked Sendable {
+    /// The leaf certificate + private key used to authenticate to the server.
+    public let secIdentity: SecIdentity
+    /// Issuer certificates to present alongside the leaf (the CA chain, leaf
+    /// excluded). May be empty when the trust anchor is already pinned server
+    /// side (nginx `ssl_client_certificate` = the device-CA) — the leaf alone
+    /// then verifies at `ssl_verify_depth 1`.
+    public let issuerCertificates: [SecCertificate]
+
+    public init(secIdentity: SecIdentity, issuerCertificates: [SecCertificate] = []) {
+        self.secIdentity = secIdentity
+        self.issuerCertificates = issuerCertificates
+    }
+
+    /// The `URLCredential` handed back to a `ClientCertificate` auth challenge.
+    ///
+    /// `.forSession` (not `.permanent`) per plan §C-iOS-1: the identity already
+    /// lives in the app's keychain item — persisting the credential in the
+    /// shared URL credential store would be a second, unmanaged copy.
+    public func urlCredential(
+        persistence: URLCredential.Persistence = .forSession
+    ) -> URLCredential {
+        URLCredential(
+            identity: secIdentity,
+            certificates: issuerCertificates.isEmpty ? nil : issuerCertificates,
+            persistence: persistence
+        )
+    }
+
+    /// The leaf `SecCertificate` backing this identity (for display / summary).
+    public func leafCertificate() -> SecCertificate? {
+        var certificate: SecCertificate?
+        let status = SecIdentityCopyCertificate(secIdentity, &certificate)
+        return status == errSecSuccess ? certificate : nil
+    }
+
+    /// Human-readable summary of the leaf certificate (subject CN, issuer CN,
+    /// expiry) for the install/rotation UI. `nil` only if the leaf can't be
+    /// read (should never happen for a valid import).
+    public func summary() -> ClientCertificateSummary? {
+        leafCertificate().map(CertificateInspector.summary(of:))
+    }
+}