Add textbook reader, exercise grading, stem-change toggle, extraction pipeline

Major changes:
- Textbook UI: chapter list, reader, and interactive exercise view (keyboard
  + Apple Pencil) surfaced under the Course tab. 30 chapters, 251 exercises.
- Stem-change conjugation toggle on Week 4 flashcard decks (E-IE, E-I, O-UE).
  Uses existing VerbForm + IrregularSpan data to render highlighted present
  tense conjugations inline.
- Deterministic on-device answer grader with partial credit (correct / close
  for accent-stripped or single-char-typo / wrong). 11 unit tests cover it.
- SharedModels: TextbookChapter (local), TextbookExerciseAttempt (cloud-
  synced), AnswerGrader helpers. Bumped schema.
- DataLoader: textbook seeder (version 8) + refresh helpers that preserve
  LanGo course decks when textbook data is re-seeded.
- Local extraction pipeline in Conjuga/Scripts/textbook/ — XHTML chapter
  parser, answer-key parser, macOS Vision image OCR + PDF page OCR, merger,
  NSSpellChecker validator, language-aware auto-fixer, and repair pass that
  re-pairs quarantined vocab rows using bounding-box coordinates.
- UI test target (ConjugaUITests) with three tests: end-to-end textbook
  flow, all-chapters screenshot audit, and stem-change toggle verification.

Generated textbook content (textbook_data.json, textbook_vocab.json) and
third-party source files are gitignored — re-run Scripts/textbook/run_pipeline.sh
locally to regenerate.

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

Conjuga/ConjugaUITests/AllChaptersScreenshotTests.swift

@@ -0,0 +1,95 @@
+import XCTest
+
+/// Screenshot every chapter of the textbook — one top + one bottom frame each —
+/// so you can visually audit parsing / rendering issues across all 30 chapters.
+final class AllChaptersScreenshotTests: XCTestCase {
+
+    override func setUpWithError() throws {
+        continueAfterFailure = true
+    }
+
+    func testScreenshotEveryChapter() throws {
+        let app = XCUIApplication()
+        app.launchArguments += ["-onboardingComplete", "YES"]
+        app.launch()
+
+        let courseTab = app.tabBars.buttons["Course"]
+        XCTAssertTrue(courseTab.waitForExistence(timeout: 5))
+        courseTab.tap()
+
+        let textbookRow = app.buttons.containing(NSPredicate(
+            format: "label CONTAINS[c] 'Complete Spanish'"
+        )).firstMatch
+        XCTAssertTrue(textbookRow.waitForExistence(timeout: 5))
+        textbookRow.tap()
+
+        // NOTE: SwiftUI List preserves scroll position across navigation pushes,
+        // so visiting chapters in-order means the next one is already visible
+        // after we return from the previous one. No need to reset.
+        attach(app, name: "00-chapter-list-top")
+
+        for chapter in 1...30 {
+            guard let row = findChapterRow(app: app, chapter: chapter) else {
+                XCTFail("Chapter \(chapter) row not reachable")
+                continue
+            }
+            row.tap()
+
+            // Chapter body — wait until the chapter's title appears as a nav bar label
+            _ = app.navigationBars.firstMatch.waitForExistence(timeout: 3)
+
+            attach(app, name: String(format: "ch%02d-top", chapter))
+            // One big scroll to sample the bottom of the chapter
+            dragFullScreen(app, direction: .up)
+            dragFullScreen(app, direction: .up)
+            attach(app, name: String(format: "ch%02d-bottom", chapter))
+
+            tapNavBack(app)
+            // Small settle wait
+            _ = app.navigationBars.firstMatch.waitForExistence(timeout: 2)
+        }
+    }
+
+    // MARK: - Helpers
+
+    private enum DragDirection { case up, down }
+
+    private func dragFullScreen(_ app: XCUIApplication, direction: DragDirection) {
+        let top = app.coordinate(withNormalizedOffset: CGVector(dx: 0.5, dy: 0.12))
+        let bot = app.coordinate(withNormalizedOffset: CGVector(dx: 0.5, dy: 0.88))
+        switch direction {
+        case .up:   bot.press(forDuration: 0.1, thenDragTo: top)
+        case .down: top.press(forDuration: 0.1, thenDragTo: bot)
+        }
+    }
+
+    private func findChapterRow(app: XCUIApplication, chapter: Int) -> XCUIElement? {
+        // Chapter row accessibility label: "<n>, <title>, ..." (SwiftUI composes
+        // label from inner Texts). Match by starting number.
+        let predicate = NSPredicate(format: "label BEGINSWITH %@", "\(chapter),")
+        let row = app.buttons.matching(predicate).firstMatch
+
+        if row.exists && row.isHittable { return row }
+
+        // Scroll down up to 8 times searching for the row — chapters visited
+        // in order, so usually 0–2 swipes suffice.
+        for _ in 0..<8 {
+            if row.exists && row.isHittable { return row }
+            dragFullScreen(app, direction: .up)
+        }
+        return row.exists ? row : nil
+    }
+
+    private func tapNavBack(_ app: XCUIApplication) {
+        let back = app.navigationBars.buttons.firstMatch
+        if back.exists && back.isHittable { back.tap() }
+    }
+
+    private func attach(_ app: XCUIApplication, name: String) {
+        let screenshot = app.screenshot()
+        let attachment = XCTAttachment(screenshot: screenshot)
+        attachment.name = name
+        attachment.lifetime = .keepAlways
+        add(attachment)
+    }
+}