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>

Sources/SstData/DictionaryStore.swift

@@ -14,6 +14,8 @@ public final class DictionaryStore {
     public let fileURL: URL
     public private(set) var file = DictionaryFile()
     public var onChange: (() -> Void)?
+    /// Fired (after `onChange`) when the file was changed by something other than this store.
+    public var onExternalChange: (() -> Void)?
 
     private var lastText = ""
     private var watcher: DispatchSourceFileSystemObject?
@@ -53,8 +55,9 @@ public final class DictionaryStore {
     public func reloadIfChanged() {
         guard let text = try? String(contentsOf: fileURL, encoding: .utf8), text != lastText else { return }
         lastText = text
-        file = DictionaryFile(parsing: text)
+        file = DictionaryFile(parsing: text).preservingIDs(from: file)
         onChange?()
+        onExternalChange?()
     }
 
     /// Watches the folder (editors replace files instead of editing them in place).