Add 13 new grammar notes with 1010 exercises from video extraction

Scraped a 4h Spanish fundamentals YouTube video (transcript + OCR on
14810 frames), extracted structured content across 52 chapters, and
generated fill-in-the-blank quizzes for every grammar topic.

- 13 new GrammarNote entries (articles, possessives, demonstratives,
  greetings, poder, al/del, prepositional pronouns, irregular yo,
  stem-changing, stressed possessives, present/future perfect, present
  indicative conjugation)
- 1010 generated exercises across all 36 grammar notes (new + existing)
- Fix tense guide parser to handle unnumbered *Usages* blocks
- Rewrite 6 broken tense guide bodies (imperative, subj pluperfect,
  subj future) with numbered usage format
- Bump courseDataVersion 5→6 with TenseGuide refresh on upgrade
- Add docs/spanish-fundamentals/ with raw transcripts, polished notes,
  structured JSON, and exercise data

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/spanish-fundamentals/exercises/seed/al-del-contractions.json

@@ -0,0 +1,58 @@
+[
+  {
+    "sentence": "Yo voy _____ baño.",
+    "correct": "al",
+    "distractor": "a el",
+    "explanation": "a + el always contracts to al.",
+    "source_chapter": "15",
+    "source_title": "al & del"
+  },
+  {
+    "sentence": "Vengo _____ mercado.",
+    "correct": "del",
+    "distractor": "de el",
+    "explanation": "de + el always contracts to del.",
+    "source_chapter": "15",
+    "source_title": "al & del"
+  },
+  {
+    "sentence": "Quiero ir _____ clase.",
+    "correct": "a la",
+    "distractor": "al",
+    "explanation": "Feminine la does not contract.",
+    "source_chapter": "15",
+    "source_title": "al & del"
+  },
+  {
+    "sentence": "Es el regalo _____ profesor.",
+    "correct": "del",
+    "distractor": "de la",
+    "explanation": "Profesor is masculine, so de + el = del.",
+    "source_chapter": "15",
+    "source_title": "al & del"
+  },
+  {
+    "sentence": "El libro es _____ él.",
+    "correct": "de",
+    "distractor": "del",
+    "explanation": "The pronoun él (with accent) does not contract; only the article el does.",
+    "source_chapter": "15",
+    "source_title": "al & del"
+  },
+  {
+    "sentence": "Vamos _____ playa.",
+    "correct": "a la",
+    "distractor": "al",
+    "explanation": "Playa is feminine; no contraction.",
+    "source_chapter": "15",
+    "source_title": "al & del"
+  },
+  {
+    "sentence": "El presidente _____ Salvador habló.",
+    "correct": "de El",
+    "distractor": "del",
+    "explanation": "Proper nouns with capital El don't contract.",
+    "source_chapter": "15",
+    "source_title": "al & del"
+  }
+]