fix: history load notifies and never overwrites unreadable file; dictionary keeps ids across external edits, undo safety

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Tests/SstDataTests/HistoryTests.swift

@@ -57,6 +57,35 @@ struct HistoryStoreTests {
         #expect(store.entries.isEmpty)
     }
 
+    @Test func loadNotifiesOnChange() {
+        let url = tempFile()
+        let writer = HistoryStore(fileURL: url)
+        writer.add(entry("a", at: 0)); writer.add(entry("b", at: 1))
+        writer.flush()
+
+        let store = HistoryStore(fileURL: url)
+        var calls = 0
+        store.onChange = { calls += 1 }
+        #expect(store.load() == .loaded)
+        #expect(calls == 1)
+        #expect(store.entries.count == 2)
+    }
+
+    @Test func unreadableFileIsReportedAndNeverOverwritten() throws {
+        let url = tempFile()
+        try FileManager.default.createDirectory(at: url, withIntermediateDirectories: true)   // history.json is a directory
+        let store = HistoryStore(fileURL: url)
+        guard case .unreadable = store.load() else {
+            Issue.record("expected unreadable"); return
+        }
+        store.add(entry("a", at: 0))
+        store.flush()
+        var isDirectory: ObjCBool = false
+        #expect(FileManager.default.fileExists(atPath: url.path, isDirectory: &isDirectory))
+        #expect(isDirectory.boolValue)
+        #expect(store.lastError != nil)
+    }
+
     @Test func onChangeFiresForEdits() {
         let store = HistoryStore(fileURL: tempFile())
         var calls = 0