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/Package.swift

@@ -0,0 +1,22 @@
+// swift-tools-version: 6.0
+// C-iOS-1 · Device client-certificate (mutual-TLS) leaf package.
+//
+// Standalone, no local-package dependencies — imports ONLY Security/Foundation.
+// It is a downward leaf: SessionCore and the App target may depend on it, it
+// depends on nothing in this workspace. This keeps the pure, unit-testable
+// mTLS logic (identity wrapper, PKCS#12 import, challenge responder) isolated
+// from the WS/HTTP transport plumbing that consumes it.
+import PackageDescription
+
+let package = Package(
+    name: "ClientTLS",
+    platforms: [.iOS(.v17), .macOS(.v14)],
+    products: [
+        .library(name: "ClientTLS", targets: ["ClientTLS"]),
+    ],
+    targets: [
+        .target(name: "ClientTLS"),
+        .testTarget(name: "ClientTLSTests", dependencies: ["ClientTLS"]),
+    ],
+    swiftLanguageModes: [.v6]
+)