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/project.yml

@@ -31,6 +31,8 @@ packages:
     path: Packages/HostRegistry
   APIClient:
     path: Packages/APIClient
+  ClientTLS:
+    path: Packages/ClientTLS      # C-iOS · device client-cert (mTLS) leaf package
   TestSupport:
     path: Packages/TestSupport   # test doubles — WebTermTests only, never the app target
   # SwiftTerm is the ONLY third-party dependency, attached to the App target
@@ -50,6 +52,7 @@ targets:
       - package: SessionCore
       - package: HostRegistry
       - package: APIClient
+      - package: ClientTLS
       - package: SwiftTerm
     settings:
       base:
@@ -119,6 +122,7 @@ targets:
       - package: SessionCore
       - package: HostRegistry
       - package: APIClient
+      - package: ClientTLS
       - package: TestSupport
     settings:
       base: