Fix textbook section missing on installs that predate bundled JSON

Earlier launches (before cd491bd bundled textbook_data.json) ran
seedTextbookData, got "not bundled — skipping", and still bumped
UserDefaults textbookDataVersion to 9. Subsequent launches then
short-circuited in refreshTextbookDataIfNeeded and never re-seeded,
so ZTEXTBOOKCHAPTER stayed empty and the Course tab hid the section.

- seedTextbookData now returns Bool (true only when chapters inserted).
- Both call sites only write the version key on success, so a missing
  or unparseable bundle no longer locks us out of future retries.
- Bumped textbookDataVersion to 10 to force existing installs to
  re-seed on next launch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Conjuga/Conjuga/Services/DataLoader.swift

@@ -6,7 +6,7 @@ actor DataLoader {
     static let courseDataVersion = 7
     static let courseDataKey = "courseDataVersion"
 
-    static let textbookDataVersion = 9
+    static let textbookDataVersion = 10
     static let textbookDataKey = "textbookDataVersion"
 
     /// Quick check: does the DB need seeding or course data refresh?
@@ -140,10 +140,13 @@ actor DataLoader {
         // Seed course data (uses the same mainContext so @Query sees it)
         seedCourseData(context: context)
 
-        // Seed textbook data
+        // Seed textbook data — only bump the version key if the seed
-        seedTextbookData(context: context)
+        // actually inserted rows, so a missing/unparseable bundle doesn't
+        // permanently lock us out of future re-seeds.
+        if seedTextbookData(context: context) {
             UserDefaults.standard.set(textbookDataVersion, forKey: textbookDataKey)
         }
+    }
 
     /// Re-seed textbook data if the version has changed.
     static func refreshTextbookDataIfNeeded(container: ModelContainer) async {
@@ -165,9 +168,12 @@ actor DataLoader {
         }
         try? context.save()
 
-        seedTextbookData(context: context)
+        if seedTextbookData(context: context) {
             shared.set(textbookDataVersion, forKey: textbookDataKey)
             print("Textbook data re-seeded to version \(textbookDataVersion)")
+        } else {
+            print("Textbook re-seed failed — leaving version key untouched so next launch retries")
+        }
     }
 
     /// Re-seed course data if the version has changed (e.g. examples were added).
@@ -378,21 +384,22 @@ actor DataLoader {
 
     // MARK: - Textbook seeding
 
-    private static func seedTextbookData(context: ModelContext) {
+    @discardableResult
+    private static func seedTextbookData(context: ModelContext) -> Bool {
         let url = Bundle.main.url(forResource: "textbook_data", withExtension: "json")
             ?? Bundle.main.bundleURL.appendingPathComponent("textbook_data.json")
         guard let data = try? Data(contentsOf: url) else {
             print("[DataLoader] textbook_data.json not bundled — skipping textbook seed")
-            return
+            return false
         }
         guard let json = try? JSONSerialization.jsonObject(with: data) as? [String: Any] else {
             print("[DataLoader] ERROR: Could not parse textbook_data.json")
-            return
+            return false
         }
         let courseName = (json["courseName"] as? String) ?? "Textbook"
         guard let chapters = json["chapters"] as? [[String: Any]] else {
             print("[DataLoader] ERROR: textbook_data.json missing chapters")
-            return
+            return false
         }
 
         var inserted = 0
@@ -480,6 +487,7 @@ actor DataLoader {
         seedTextbookVocabDecks(context: context, courseName: courseName)
 
         print("Textbook seeding complete: \(inserted) chapters")
+        return inserted > 0
     }
 
     private static func seedTextbookVocabDecks(context: ModelContext, courseName: String) {