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/App/WebTerm/Wiring/AppEnvironment.swift

@@ -1,4 +1,5 @@
 import APIClient
+import ClientTLS
 import Foundation
 import HostRegistry
 import SessionCore
@@ -37,8 +38,20 @@ struct AppEnvironment: Sendable {
     var unreadStore: any UnreadWatermarkStore = UserDefaultsUnreadWatermarkStore()
 
     static func production() -> AppEnvironment {
-        let http = URLSessionHTTPTransport()
-        let termTransport = URLSessionTermTransport()
+        // C-iOS-2 (MEDIUM no-relaunch fix) · Resolve the installed device client
+        // identity LAZILY from the keychain on each connect/challenge, injected
+        // into BOTH transports + the probe as a provider. This way a certificate
+        // imported from the "设备证书" screen takes effect on the NEXT connection
+        // without relaunching — a snapshot captured here would stay stale. A
+        // missing cert is the normal pre-install state (→ nil); genuine faults
+        // are logged, not fatal (`loadedIdentityOrNil`). mTLS challenges only
+        // fire for tunnel hosts, so a `nil` result is inert for local hosts.
+        let identityStore = KeychainClientIdentityStore()
+        let identityProvider: @Sendable () -> ClientIdentity? = {
+            identityStore.loadedIdentityOrNil()
+        }
+        let http = URLSessionHTTPTransport(identityProvider: identityProvider)
+        let termTransport = URLSessionTermTransport(identityProvider: identityProvider)
         return AppEnvironment(
             hostStore: KeychainHostStore(),
             lastSessionStore: UserDefaultsLastSessionStore(),