Rewrite all UI tests following fail-fast TEST_RULES patterns

Rewrote 60+ test files to follow honeydue-style test guidelines:
- defaultTimeout=2s, navigationTimeout=5s — fail fast, no long waits
- No coordinate taps (except onboarding paged TabView swipes)
- No sleep(), no retry loops
- No guard...else { return } silent passes — XCTFail everywhere
- All elements by accessibility ID via UITestID constants
- Screen objects for all navigation/actions/assertions
- One logical assertion per test method

Added missing accessibility identifiers to app views:
- MonthView.swift: added AccessibilityID.MonthView.grid to ScrollView
- YearView.swift: added AccessibilityID.YearView.heatmap to ScrollView

Framework rewrites:
- BaseUITestCase: added session ID, localeArguments, extraLaunchArguments
- WaitHelpers: waitForExistenceOrFail, waitUntilHittableOrFail,
  waitForNonExistence, scrollIntoView, forceTap
- All 7 screen objects rewritten with fail-fast semantics
- TEST_RULES.md added with non-negotiable rules

Known remaining issues:
- OnboardingTests: paged TabView swipes unreliable on iOS 26 simulator
- SettingsLegalLinksTests: EULA/Privacy buttons too deep in DEBUG scroll
- Customization horizontal picker scrolling needs further tuning

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Tests iOS/AppLaunchTests.swift

@@ -8,54 +8,50 @@
 import XCTest
 
 final class AppLaunchTests: BaseUITestCase {
-    override var seedFixture: String? { "empty" }
+    override var seedFixture: String? { "week_of_moods" }
 
-    /// Verify the app launches to the Day tab and all 5 tabs are visible.
+    /// Verify the app launches and the tab bar is visible.
     func testAppLaunches_TabBarVisible() {
         let tabBar = TabBarScreen(app: app)
-        tabBar.assertTabBarVisible()
-
-        // All 5 tabs should exist
-        XCTAssertTrue(tabBar.dayTab.exists, "Day tab should exist")
-        XCTAssertTrue(tabBar.monthTab.exists, "Month tab should exist")
-        XCTAssertTrue(tabBar.yearTab.exists, "Year tab should exist")
-        XCTAssertTrue(tabBar.insightsTab.exists, "Insights tab should exist")
-        XCTAssertTrue(tabBar.settingsTab.exists, "Settings tab should exist")
-
-        captureScreenshot(name: "app_launched")
+        tabBar.assertVisible()
     }
 
-    /// Navigate through every tab and verify each loads.
-    func testTabNavigation_AllTabsAccessible() {
+    /// Navigate to Month tab and verify it loads.
+    func testTabNavigation_Month() {
         let tabBar = TabBarScreen(app: app)
-
-        // Month tab
         tabBar.tapMonth()
-        assertTabSelected(tabBar.monthTab, name: "Month")
-
-        // Year tab
-        tabBar.tapYear()
-        assertTabSelected(tabBar.yearTab, name: "Year")
-
-        // Insights tab
-        tabBar.tapInsights()
-        assertTabSelected(tabBar.insightsTab, name: "Insights")
-
-        // Settings tab
-        tabBar.tapSettings()
-        assertTabSelected(tabBar.settingsTab, name: "Settings")
-
-        // Back to Day
-        tabBar.tapDay()
-        assertTabSelected(tabBar.dayTab, name: "Day")
+        let monthGrid = app.element(UITestID.Month.grid)
+        monthGrid.waitForExistenceOrFail(timeout: navigationTimeout, message: "Month grid should be visible after tapping Month tab")
     }
 
-    /// Wait for a tab to become selected (iOS 26 Liquid Glass may delay state updates).
-    private func assertTabSelected(_ tab: XCUIElement, name: String, timeout: TimeInterval = 8) {
-        // Re-query the element to get fresh state, since isSelected can be stale.
-        let predicate = NSPredicate(format: "isSelected == true")
-        let expectation = XCTNSPredicateExpectation(predicate: predicate, object: tab)
-        let result = XCTWaiter.wait(for: [expectation], timeout: timeout)
-        XCTAssertEqual(result, .completed, "\(name) tab should be selected")
+    /// Navigate to Year tab and verify it loads.
+    func testTabNavigation_Year() {
+        let tabBar = TabBarScreen(app: app)
+        tabBar.tapYear()
+        let heatmap = app.element(UITestID.Year.heatmap)
+        heatmap.waitForExistenceOrFail(timeout: navigationTimeout, message: "Year heatmap should be visible after tapping Year tab")
+    }
+
+    /// Navigate to Insights tab and verify it loads.
+    func testTabNavigation_Insights() {
+        let tabBar = TabBarScreen(app: app)
+        tabBar.tapInsights()
+        let insightsHeader = app.element(UITestID.Insights.header)
+        insightsHeader.waitForExistenceOrFail(timeout: navigationTimeout, message: "Insights header should be visible after tapping Insights tab")
+    }
+
+    /// Navigate to Settings tab and verify it loads.
+    func testTabNavigation_Settings() {
+        let tabBar = TabBarScreen(app: app)
+        let settingsScreen = tabBar.tapSettings()
+        settingsScreen.assertVisible()
+    }
+
+    /// Navigate away from Day and return -- Day screen loads.
+    func testTabNavigation_ReturnToDay() {
+        let tabBar = TabBarScreen(app: app)
+        tabBar.tapSettings()
+        let dayScreen = tabBar.tapDay()
+        dayScreen.assertVisible()
     }
 }