feat: clone Opal's rules feature with hard block enforcement

Rebuild the app around recurring screen-time blocking rules modeled on
Opal's My Apps tab:

- Onboarding with FamilyControls Screen Time authorization
- Apps home with Blocked Apps tiles and live rule cards
- New Rule sheet: Schedule/Time Limit/Open Limit types + preset gallery
- Rule editors: time windows (incl. overnight), day picker, app selection
  via FamilyActivityPicker (Block/Allow Only), rename, Hold to Commit
- Hard Mode: active hard rules cannot be edited, disabled, deleted, or
  unblocked until their window ends; soft rules pause until next window
- Shield enforcement through per-rule ManagedSettingsStore + RuleEnforcer
- 73 unit tests (Swift Testing) + 16 UI tests (XCUITest) with launch-arg
  harness for in-memory storage, mocked authorization, seeded scenarios
- docs/RULES_FEATURE_SPEC.md: spec derived from the reference recording

Known gap: background window transitions and time/open-limit thresholds
need a DeviceActivityMonitor extension; shields currently sync while the
app is running.

Severed/Views/RootView.swift

@@ -0,0 +1,32 @@
+//
+//  RootView.swift
+//  Severed
+//
+
+import SwiftUI
+
+/// Gates the app on onboarding: until the user has walked through the welcome
+/// and Screen Time permission steps, nothing else is reachable.
+struct RootView: View {
+    @AppStorage("hasCompletedOnboarding") private var hasCompletedOnboarding = false
+    @Environment(ScreenTimeAuthorization.self) private var authorization
+    @Environment(\.scenePhase) private var scenePhase
+
+    var body: some View {
+        Group {
+            if hasCompletedOnboarding {
+                AppsHomeView()
+            } else {
+                OnboardingView {
+                    hasCompletedOnboarding = true
+                }
+            }
+        }
+        .onChange(of: scenePhase) { _, phase in
+            // Pick up permission changes made in Settings while we were backgrounded.
+            if phase == .active {
+                authorization.refresh()
+            }
+        }
+    }
+}