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/ocr_pdf.swift

@@ -0,0 +1,133 @@
+#!/usr/bin/env swift
+// Rasterize each page of a PDF at high DPI and OCR it with Vision.
+// Output: { "<pdfIndex>": { "lines": [...], "confidence": Double, "bookPage": Int? } }
+//
+// Usage: swift ocr_pdf.swift <pdf_path> <output_json> [dpi]
+// Example: swift ocr_pdf.swift "book.pdf" pdf_ocr.json 240
+
+import Foundation
+import Vision
+import AppKit
+import Quartz
+
+guard CommandLine.arguments.count >= 3 else {
+    print("Usage: swift ocr_pdf.swift <pdf_path> <output_json> [dpi]")
+    exit(1)
+}
+
+let pdfURL = URL(fileURLWithPath: CommandLine.arguments[1])
+let outputURL = URL(fileURLWithPath: CommandLine.arguments[2])
+let dpi: CGFloat = CommandLine.arguments.count >= 4 ? CGFloat(Double(CommandLine.arguments[3]) ?? 240.0) : 240.0
+
+guard let pdfDoc = PDFDocument(url: pdfURL) else {
+    print("Could not open PDF at \(pdfURL.path)")
+    exit(1)
+}
+
+let pageCount = pdfDoc.pageCount
+print("PDF has \(pageCount) pages. Rendering at \(dpi) DPI.")
+
+struct PageResult: Encodable {
+    var lines: [String]
+    var confidence: Double
+    var bookPage: Int?
+}
+
+var results: [String: PageResult] = [:]
+let startTime = Date()
+
+// Render at scale = dpi / 72 (72 is default PDF DPI)
+let scale: CGFloat = dpi / 72.0
+
+for i in 0..<pageCount {
+    guard let page = pdfDoc.page(at: i) else { continue }
+    let pageBounds = page.bounds(for: .mediaBox)
+    let scaledSize = CGSize(width: pageBounds.width * scale, height: pageBounds.height * scale)
+
+    // Render the page into a CGImage
+    let colorSpace = CGColorSpaceCreateDeviceRGB()
+    let bitmapInfo = CGImageAlphaInfo.noneSkipLast.rawValue
+    guard let context = CGContext(
+        data: nil,
+        width: Int(scaledSize.width),
+        height: Int(scaledSize.height),
+        bitsPerComponent: 8,
+        bytesPerRow: 0,
+        space: colorSpace,
+        bitmapInfo: bitmapInfo
+    ) else {
+        print("\(i): could not create CGContext")
+        continue
+    }
+    context.setFillColor(CGColor(gray: 1.0, alpha: 1.0))
+    context.fill(CGRect(origin: .zero, size: scaledSize))
+    context.scaleBy(x: scale, y: scale)
+    page.draw(with: .mediaBox, to: context)
+
+    guard let cgImage = context.makeImage() else {
+        print("\(i): could not create CGImage")
+        continue
+    }
+
+    let handler = VNImageRequestHandler(cgImage: cgImage, options: [:])
+    let request = VNRecognizeTextRequest()
+    request.recognitionLevel = .accurate
+    request.recognitionLanguages = ["es-ES", "es", "en-US"]
+    request.usesLanguageCorrection = true
+    if #available(macOS 13.0, *) {
+        request.automaticallyDetectsLanguage = true
+    }
+
+    do {
+        try handler.perform([request])
+        let observations = request.results ?? []
+        var lines: [String] = []
+        var totalConfidence: Float = 0
+        var count = 0
+        for obs in observations {
+            if let top = obs.topCandidates(1).first {
+                let s = top.string.trimmingCharacters(in: .whitespaces)
+                if !s.isEmpty {
+                    lines.append(s)
+                    totalConfidence += top.confidence
+                    count += 1
+                }
+            }
+        }
+        let avg = count > 0 ? Double(totalConfidence) / Double(count) : 0.0
+
+        // Try to detect book page number: a short numeric line in the first
+        // 3 or last 3 entries (typical page-number placement).
+        var bookPage: Int? = nil
+        let candidates = Array(lines.prefix(3)) + Array(lines.suffix(3))
+        for c in candidates {
+            let trimmed = c.trimmingCharacters(in: .whitespaces)
+            if let n = Int(trimmed), n >= 1 && n <= 1000 {
+                bookPage = n
+                break
+            }
+        }
+
+        results[String(i)] = PageResult(lines: lines, confidence: avg, bookPage: bookPage)
+    } catch {
+        print("\(i): \(error)")
+    }
+
+    if (i + 1) % 25 == 0 || (i + 1) == pageCount {
+        let elapsed = Date().timeIntervalSince(startTime)
+        let rate = Double(i + 1) / max(elapsed, 0.001)
+        let remaining = Double(pageCount - (i + 1)) / max(rate, 0.001)
+        print(String(format: "%d/%d  %.1f pg/s  eta %.0fs", i + 1, pageCount, rate, remaining))
+    }
+}
+
+let encoder = JSONEncoder()
+encoder.outputFormatting = [.sortedKeys]
+do {
+    let data = try encoder.encode(results)
+    try data.write(to: outputURL)
+    print("Wrote \(results.count) pages to \(outputURL.path)")
+} catch {
+    print("Error writing output: \(error)")
+    exit(1)
+}