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/SessionCore/Package.swift

@@ -1,7 +1,9 @@
 // swift-tools-version: 6.0
 // T-iOS-1 scaffold shell. SessionEngine / ReconnectMachine / PingScheduler /
 // GateState / AwayDigest / URLSessionTermTransport land in W1–W2 tasks.
-// Dependency direction is strictly downward: SessionCore → WireProtocol only.
+// Dependency direction is strictly downward: SessionCore → WireProtocol, plus
+// ClientTLS (C-iOS-2) for the device client-cert mTLS responder the WS transport
+// answers connection-level challenges with. Both are downward leaves.
 import PackageDescription
 
 let package = Package(
@@ -12,12 +14,16 @@ let package = Package(
     ],
     dependencies: [
         .package(path: "../WireProtocol"),
+        .package(path: "../ClientTLS"),
         .package(path: "../TestSupport"),
     ],
     targets: [
         .target(
             name: "SessionCore",
-            dependencies: [.product(name: "WireProtocol", package: "WireProtocol")]
+            dependencies: [
+                .product(name: "WireProtocol", package: "WireProtocol"),
+                .product(name: "ClientTLS", package: "ClientTLS"),
+            ]
         ),
         .testTarget(
             name: "SessionCoreTests",