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/Scripts/textbook/validate_vocab.swift

@@ -0,0 +1,156 @@
+#!/usr/bin/env swift
+// Validate every Spanish/English word in vocab_cards.json using NSSpellChecker.
+// For each flagged word, produce up to 3 candidate corrections.
+//
+// Usage: swift validate_vocab.swift <vocab_cards.json> <output_report.json>
+
+import Foundation
+import AppKit
+
+guard CommandLine.arguments.count >= 3 else {
+    print("Usage: swift validate_vocab.swift <vocab_cards.json> <output_report.json>")
+    exit(1)
+}
+
+let inputURL = URL(fileURLWithPath: CommandLine.arguments[1])
+let outputURL = URL(fileURLWithPath: CommandLine.arguments[2])
+
+guard let data = try? Data(contentsOf: inputURL),
+      let json = try? JSONSerialization.jsonObject(with: data) as? [String: Any],
+      let chapters = json["chapters"] as? [[String: Any]] else {
+    print("Could not load \(inputURL.path)")
+    exit(1)
+}
+
+let checker = NSSpellChecker.shared
+
+// Tokenize — only letter runs (Unicode aware for Spanish accents)
+func tokens(_ s: String) -> [String] {
+    let letters = CharacterSet.letters
+    return s.unicodeScalars
+        .split { !letters.contains($0) }
+        .map { String(String.UnicodeScalarView($0)) }
+        .filter { !$0.isEmpty }
+}
+
+// Minimal stopword set — names, proper nouns, numeric tokens already filtered
+let stopES: Set<String> = [
+    "el", "la", "los", "las", "un", "una", "unos", "unas", "del", "al", "de",
+    "a", "en", "y", "o", "que", "no", "se", "con", "por", "para", "lo", "le",
+    "su", "mi", "tu", "yo", "te", "me", "es", "son", "está", "están",
+]
+let stopEN: Set<String> = [
+    "the", "a", "an", "to", "of", "in", "and", "or", "is", "are", "was", "were",
+    "be", "been", "my", "his", "her", "our", "their", "your",
+]
+
+func checkWord(_ w: String, lang: String, stop: Set<String>) -> [String]? {
+    // Return nil if word is OK, else list of candidate corrections.
+    if w.count < 2 { return nil }
+    if stop.contains(w.lowercased()) { return nil }
+    if w.rangeOfCharacter(from: .decimalDigits) != nil { return nil }
+
+    let range = checker.checkSpelling(
+        of: w,
+        startingAt: 0,
+        language: lang,
+        wrap: false,
+        inSpellDocumentWithTag: 0,
+        wordCount: nil
+    )
+    // Range of `(0, 0)` means no misspelling; otherwise we have a misspelling.
+    if range.location == NSNotFound || range.length == 0 { return nil }
+
+    let guesses = checker.guesses(
+        forWordRange: NSRange(location: 0, length: (w as NSString).length),
+        in: w,
+        language: lang,
+        inSpellDocumentWithTag: 0
+    ) ?? []
+    return Array(guesses.prefix(3))
+}
+
+struct Flag: Encodable {
+    var chapter: Int
+    var front: String
+    var back: String
+    var badFront: [BadWord]
+    var badBack: [BadWord]
+    var sourceImage: String
+}
+struct BadWord: Encodable {
+    var word: String
+    var suggestions: [String]
+    var side: String  // "es" or "en"
+}
+
+var flags: [Flag] = []
+var totalCards = 0
+var totalBadES = 0
+var totalBadEN = 0
+
+for ch in chapters {
+    guard let chNum = ch["chapter"] as? Int,
+          let cards = ch["cards"] as? [[String: Any]] else { continue }
+    for card in cards {
+        totalCards += 1
+        let front = (card["front"] as? String) ?? ""
+        let back = (card["back"] as? String) ?? ""
+        let img = (card["sourceImage"] as? String) ?? ""
+
+        var badFront: [BadWord] = []
+        for w in tokens(front) {
+            if let sugg = checkWord(w, lang: "es", stop: stopES) {
+                badFront.append(BadWord(word: w, suggestions: sugg, side: "es"))
+                totalBadES += 1
+            }
+        }
+        var badBack: [BadWord] = []
+        for w in tokens(back) {
+            if let sugg = checkWord(w, lang: "en", stop: stopEN) {
+                badBack.append(BadWord(word: w, suggestions: sugg, side: "en"))
+                totalBadEN += 1
+            }
+        }
+        if !badFront.isEmpty || !badBack.isEmpty {
+            flags.append(Flag(
+                chapter: chNum,
+                front: front,
+                back: back,
+                badFront: badFront,
+                badBack: badBack,
+                sourceImage: img
+            ))
+        }
+    }
+}
+
+struct Report: Encodable {
+    var totalCards: Int
+    var flaggedCards: Int
+    var flaggedSpanishWords: Int
+    var flaggedEnglishWords: Int
+    var flags: [Flag]
+}
+let report = Report(
+    totalCards: totalCards,
+    flaggedCards: flags.count,
+    flaggedSpanishWords: totalBadES,
+    flaggedEnglishWords: totalBadEN,
+    flags: flags
+)
+
+let encoder = JSONEncoder()
+encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
+do {
+    let data = try encoder.encode(report)
+    try data.write(to: outputURL)
+    print("Cards:              \(totalCards)")
+    print("Flagged cards:      \(flags.count)  (\(Double(flags.count)/Double(totalCards)*100.0 as Double)%)")
+    print("Flagged ES words:   \(totalBadES)")
+    print("Flagged EN words:   \(totalBadEN)")
+    print("Wrote \(outputURL.path)")
+} catch {
+    print("Error writing output: \(error)")
+    exit(1)
+}