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>

composeApp/src/jvmMain/kotlin/com/example/casera/platform/PlatformUpgradeScreen.jvm.kt

@@ -0,0 +1,16 @@
+package com.example.casera.platform
+
+import androidx.compose.runtime.Composable
+import com.example.casera.ui.subscription.UpgradeScreen
+
+@Composable
+actual fun PlatformUpgradeScreen(
+    onNavigateBack: () -> Unit,
+    onSubscriptionChanged: () -> Unit
+) {
+    UpgradeScreen(
+        onNavigateBack = onNavigateBack,
+        onPurchase = { _ -> onNavigateBack() },
+        onRestorePurchases = { }
+    )
+}