Complete re-validation remediation: KMP architecture, iOS platform, XCUITest rewrite

Phases 1-6 of fixes.md — closes all 13 issues from codex_issues_2.md re-validation:

KMP Architecture:
- Fix subscription purchase/restore response contract (VerificationResponse aligned)
- Add feature benefits auth token + APILayer init flow
- Remove ResidenceFormScreen direct API bypass (use APILayer)
- Wire paywall purchase/restore to real SubscriptionApi calls

iOS Platform:
- Add iOS Keychain token storage via Swift KeychainHelper
- Implement Google Sign-In via ASWebAuthenticationSession (GoogleSignInManager)
- DocumentViewModelWrapper observes DataManager for auto-updates
- Add missing accessibility identifiers (document, task columns, Google Sign-In)

XCUITest Rewrite:
- Rewrite test infrastructure: zero sleep() calls, accessibility ID lookups
- Create AuthCriticalPathTests and NavigationCriticalPathTests
- Delete 14 legacy brittle test files (Suite0-10, templates)
- Fix CaseraTests module import (@testable import Casera)

All platforms build clean. TEST BUILD SUCCEEDED.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

iosApp/CaseraUITests/AccessibilityIdentifiers.swift

@@ -13,6 +13,7 @@ struct AccessibilityIdentifiers {
         static let forgotPasswordButton = "Login.ForgotPasswordButton"
         static let passwordVisibilityToggle = "Login.PasswordVisibilityToggle"
         static let appleSignInButton = "Login.AppleSignInButton"
+        static let googleSignInButton = "Login.GoogleSignInButton"
 
         // Registration
         static let registerUsernameField = "Register.UsernameField"
@@ -35,6 +36,7 @@ struct AccessibilityIdentifiers {
         static let contractorsTab = "TabBar.Contractors"
         static let documentsTab = "TabBar.Documents"
         static let profileTab = "TabBar.Profile"
+        static let settingsButton = "Navigation.SettingsButton"
         static let backButton = "Navigation.BackButton"
     }