Use one observable source for Changes tab preferences

mac/Sources/Byre/Changes.swift

@@ -22,7 +22,9 @@ private let emptyText: [ChangeMode: String] = [
     .step: "This snapshot changed nothing.",
 ]
 
-struct ChangesMemory {
+@MainActor
+@Observable
+final class ChangesMemory {
     var open: Set<String> = []
     var range = ChangeMode.main
     var pinRange = ChangeMode.step
@@ -41,9 +43,7 @@ final class ChangesModel {
     private(set) var changes: TaskChanges?
     private(set) var error = ""
     private(set) var diffs: [String: Result<String, ApiError>] = [:]
-    private(set) var open: Set<String>
-    private(set) var range: ChangeMode
-    private(set) var pinRange: ChangeMode
+    private let memory: ChangesMemory
     @ObservationIgnored private var loading = false
     @ObservationIgnored private var again = false
     @ObservationIgnored private var timer: Task<Void, Never>?
@@ -53,11 +53,13 @@ final class ChangesModel {
 
     init(_ store: Store) {
         self.store = store
-        open = store.changesMemory.open
-        range = store.changesMemory.range
-        pinRange = store.changesMemory.pinRange
+        memory = store.changesMemory
     }
 
+    var open: Set<String> { memory.open }
+    var range: ChangeMode { memory.range }
+    var pinRange: ChangeMode { memory.pinRange }
+
     var pinned: Int? {
         store.pinned
     }
@@ -160,8 +162,7 @@ final class ChangesModel {
         if range == r {
             return
         }
-        range = r
-        store.changesMemory.range = r
+        memory.range = r
         retarget()
     }
 
@@ -169,18 +170,16 @@ final class ChangesModel {
         if pinRange == r {
             return
         }
-        pinRange = r
-        store.changesMemory.pinRange = r
+        memory.pinRange = r
         retarget()
     }
 
     func setOpen(_ path: String, _ on: Bool) {
         if on {
-            open.insert(path)
+            memory.open.insert(path)
         } else {
-            open.remove(path)
+            memory.open.remove(path)
         }
-        store.changesMemory.open = open
     }
 
     func shows(_ a: LineAnchor) -> Bool {