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.

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

Severed/Models/RuleKind.swift

@@ -0,0 +1,64 @@
+//
+//  RuleKind.swift
+//  Severed
+//
+
+import Foundation
+
+/// The three kinds of blocking rules, mirroring the reference app's "New Rule" sheet.
+enum RuleKind: String, Codable, CaseIterable, Sendable {
+    /// Block selected apps during a recurring time window.
+    case schedule
+    /// Block selected apps after a daily usage budget is spent.
+    case timeLimit
+    /// Block selected apps after a number of opens per day.
+    case openLimit
+
+    var displayName: String {
+        switch self {
+        case .schedule: "Schedule"
+        case .timeLimit: "Time Limit"
+        case .openLimit: "Open Limit"
+        }
+    }
+
+    var exampleText: String {
+        switch self {
+        case .schedule: "e.g. 9-5, Daily"
+        case .timeLimit: "e.g. 45m/day"
+        case .openLimit: "e.g. 5 opens/day"
+        }
+    }
+
+    var symbolName: String {
+        switch self {
+        case .schedule: "calendar"
+        case .timeLimit: "hourglass"
+        case .openLimit: "lock.fill"
+        }
+    }
+
+    /// Default name given to a brand-new rule of this kind (Opal: "In the Zone", "Time Keeper").
+    var defaultRuleName: String {
+        switch self {
+        case .schedule: "In the Zone"
+        case .timeLimit: "Time Keeper"
+        case .openLimit: "Gate Keeper"
+        }
+    }
+}
+
+/// How the rule's app selection is interpreted.
+enum SelectionMode: String, Codable, CaseIterable, Sendable {
+    /// Block the selected apps; everything else stays available.
+    case block
+    /// Block everything except the selected apps.
+    case allowOnly
+
+    var displayName: String {
+        switch self {
+        case .block: "Block"
+        case .allowOnly: "Allow Only"
+        }
+    }
+}