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/Tests/ClientTLSTests/ClientIdentityStoreTests.swift

@@ -0,0 +1,62 @@
+import Foundation
+import Testing
+@testable import ClientTLS
+
+// C-iOS-1 · Store roundtrip via the InMemory store (same import/summary code
+// path as the keychain store, minus the entitlement — the keychain variant is
+// covered by on-device/simulator tests, mirroring KeychainHostStoreLiveTests).
+
+@Test("save → loadIdentity roundtrips and hasInstalledIdentity flips")
+func storeRoundtrip() throws {
+    // Arrange
+    let store = InMemoryClientIdentityStore()
+    #expect(store.hasInstalledIdentity() == false)
+    #expect(try store.loadIdentity() == nil)
+
+    // Act
+    try store.save(
+        p12Data: ClientTLSFixtures.deviceP12Data, passphrase: ClientTLSFixtures.passphrase
+    )
+
+    // Assert
+    #expect(store.hasInstalledIdentity() == true)
+    let identity = try #require(try store.loadIdentity())
+    #expect(identity.summary()?.subjectCommonName == ClientTLSFixtures.leafCommonName)
+    let summary = try #require(try store.loadSummary())
+    #expect(summary.issuerCommonName == ClientTLSFixtures.issuerCommonName)
+}
+
+@Test("save validates the passphrase and leaves prior state untouched on failure")
+func storeSaveValidatesBeforePersisting() {
+    // Arrange
+    let store = InMemoryClientIdentityStore()
+
+    // Act / Assert — a wrong passphrase must surface, not persist.
+    #expect(throws: PKCS12ImportError.wrongPassphrase) {
+        try store.save(p12Data: ClientTLSFixtures.deviceP12Data, passphrase: "wrong")
+    }
+    #expect(store.hasInstalledIdentity() == false)
+}
+
+@Test("remove clears the stored identity")
+func storeRemove() throws {
+    // Arrange
+    let store = InMemoryClientIdentityStore()
+    try store.save(
+        p12Data: ClientTLSFixtures.deviceP12Data, passphrase: ClientTLSFixtures.passphrase
+    )
+    #expect(store.hasInstalledIdentity() == true)
+
+    // Act
+    try store.remove()
+
+    // Assert
+    #expect(store.hasInstalledIdentity() == false)
+    #expect(try store.loadIdentity() == nil)
+}
+
+@Test("loadedIdentityOrNil returns nil (not a throw) when nothing is installed")
+func loadedIdentityOrNilEmpty() {
+    let store = InMemoryClientIdentityStore()
+    #expect(store.loadedIdentityOrNil() == nil)
+}