feat: adult content filter toggle + native New Rule navigation

- Add Block Adult Content toggle to all three rule editors, persisted as
  BlockingRule.blockAdultContent (inline default for clean migration of
  existing stores) and surfaced in the detail sheet as an
  'Adult websites: Blocked/Allowed' row
- Engage Screen Time's adult-website filter
  (webContent.blockedByFilter = .auto()) alongside the rule's shield and
  clear it when the shield clears
- Replace the New Rule sheet's view-swap with a NavigationStack push
  (navigationDestination(item:)); the editor uses native chrome there
  (system back button, inline title, toolbar rename), enabling the push
  animation and edge-swipe back
- Tests: 93 passing (+2 unit: draft round-trip and enforcer forwarding;
  +3 UI: toggle-to-detail flow, default-allowed row, swipe-back as a
  behavioral proof of native navigation)
- Spec updated accordingly (editor sections, behavior, data model,
  navigation note)

Severed/Models/RuleDraft.swift

@@ -6,8 +6,9 @@
 import Foundation
 
 /// Value-type working copy of a rule used by the editors, so cancelling an
-/// edit never touches the persisted model.
-struct RuleDraft: Equatable {
+/// edit never touches the persisted model. Hashable so it can drive
+/// `navigationDestination(item:)`.
+struct RuleDraft: Hashable {
     var name: String
     var kind: RuleKind
     var days: Set<Weekday>
@@ -16,6 +17,7 @@ struct RuleDraft: Equatable {
     var dailyLimitMinutes: Int
     var maxOpens: Int
     var hardMode: Bool
+    var blockAdultContent: Bool
     var selectionMode: SelectionMode
     var selectionData: Data?
     var selectionCount: Int
@@ -31,6 +33,7 @@ struct RuleDraft: Equatable {
         self.dailyLimitMinutes = 45
         self.maxOpens = 5
         self.hardMode = false
+        self.blockAdultContent = false
         self.selectionMode = .block
         self.selectionData = nil
         self.selectionCount = 0
@@ -45,6 +48,7 @@ struct RuleDraft: Equatable {
         self.dailyLimitMinutes = rule.dailyLimitMinutes
         self.maxOpens = rule.maxOpens
         self.hardMode = rule.hardMode
+        self.blockAdultContent = rule.blockAdultContent
         self.selectionMode = rule.selectionMode
         self.selectionData = rule.selectionData
         self.selectionCount = rule.selectionCount
@@ -67,6 +71,7 @@ struct RuleDraft: Equatable {
         rule.dailyLimitMinutes = dailyLimitMinutes
         rule.maxOpens = maxOpens
         rule.hardMode = hardMode
+        rule.blockAdultContent = blockAdultContent
         rule.selectionMode = selectionMode
         rule.selectionData = selectionData
         rule.selectionCount = selectionCount