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/SharedModels/Tests/SharedModelsTests/AnswerGraderTests.swift

@@ -0,0 +1,80 @@
+import Testing
+@testable import SharedModels
+
+@Suite("AnswerGrader")
+struct AnswerGraderTests {
+
+    @Test("exact match is correct")
+    func exact() {
+        #expect(AnswerGrader.grade(userText: "tengo", canonical: "tengo") == .correct)
+        #expect(AnswerGrader.grade(userText: "Tengo", canonical: "tengo") == .correct)
+        #expect(AnswerGrader.grade(userText: "  tengo  ", canonical: "tengo") == .correct)
+    }
+
+    @Test("missing accent is close")
+    func missingAccent() {
+        #expect(AnswerGrader.grade(userText: "esta", canonical: "está") == .close)
+        #expect(AnswerGrader.grade(userText: "nino", canonical: "niño") == .close)
+        #expect(AnswerGrader.grade(userText: "asi", canonical: "así") == .close)
+    }
+
+    @Test("single-char typo is close")
+    func singleCharTypo() {
+        // deletion
+        #expect(AnswerGrader.grade(userText: "tngo", canonical: "tengo") == .close)
+        // insertion
+        #expect(AnswerGrader.grade(userText: "tengoo", canonical: "tengo") == .close)
+        // substitution
+        #expect(AnswerGrader.grade(userText: "tengu", canonical: "tengo") == .close)
+    }
+
+    @Test("two-char typo is wrong")
+    func twoCharTypo() {
+        #expect(AnswerGrader.grade(userText: "tngu", canonical: "tengo") == .wrong)
+    }
+
+    @Test("empty is wrong")
+    func empty() {
+        #expect(AnswerGrader.grade(userText: "", canonical: "tengo") == .wrong)
+        #expect(AnswerGrader.grade(userText: "   ", canonical: "tengo") == .wrong)
+    }
+
+    @Test("alternates accepted")
+    func alternates() {
+        #expect(AnswerGrader.grade(userText: "flaca", canonical: "delgada", alternates: ["flaca"]) == .correct)
+        #expect(AnswerGrader.grade(userText: "flacca", canonical: "delgada", alternates: ["flaca"]) == .close)
+    }
+
+    @Test("punctuation stripped")
+    func punctuation() {
+        #expect(AnswerGrader.grade(userText: "el libro.", canonical: "el libro") == .correct)
+        #expect(AnswerGrader.grade(userText: "¿dónde?", canonical: "dónde") == .correct)
+    }
+
+    @Test("very different text is wrong")
+    func wrong() {
+        #expect(AnswerGrader.grade(userText: "hola", canonical: "tengo") == .wrong)
+        #expect(AnswerGrader.grade(userText: "casa", canonical: "perro") == .wrong)
+    }
+
+    @Test("normalize produces expected output")
+    func normalize() {
+        #expect(AnswerGrader.normalize("  Hola  ") == "hola")
+        #expect(AnswerGrader.normalize("ABC!") == "abc")
+    }
+
+    @Test("stripAccents handles common Spanish diacritics")
+    func stripAccents() {
+        #expect(AnswerGrader.stripAccents("niño") == "nino")
+        #expect(AnswerGrader.stripAccents("está") == "esta")
+        #expect(AnswerGrader.stripAccents("güero") == "guero")
+    }
+
+    @Test("levenshtein computes edit distance")
+    func levenshtein() {
+        #expect(AnswerGrader.levenshtein("kitten", "sitting") == 3)
+        #expect(AnswerGrader.levenshtein("flaw", "lawn") == 2)
+        #expect(AnswerGrader.levenshtein("abc", "abc") == 0)
+        #expect(AnswerGrader.levenshtein("", "abc") == 3)
+    }
+}