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/greetings-farewells.json

@@ -0,0 +1,50 @@
+[
+  {
+    "sentence": "¿Cómo _____ tú?",
+    "correct": "estás",
+    "distractor": "eres",
+    "explanation": "Well-being is temporary, so use estar.",
+    "source_chapter": "11",
+    "source_title": "Useful Greetings & Farewells"
+  },
+  {
+    "sentence": "(Yo) _____ bien.",
+    "correct": "estoy",
+    "distractor": "soy",
+    "explanation": "Reply matches the verb in the question (estar).",
+    "source_chapter": "11",
+    "source_title": "Useful Greetings & Farewells"
+  },
+  {
+    "sentence": "_____ días. (good morning)",
+    "correct": "Buenos",
+    "distractor": "Buenas",
+    "explanation": "Día is masculine, so the plural adjective is buenos.",
+    "source_chapter": "11",
+    "source_title": "Useful Greetings & Farewells"
+  },
+  {
+    "sentence": "_____ noches. (good night)",
+    "correct": "Buenas",
+    "distractor": "Buenos",
+    "explanation": "Noche is feminine → buenas.",
+    "source_chapter": "11",
+    "source_title": "Useful Greetings & Farewells"
+  },
+  {
+    "sentence": "Thanks → 'Muchas _____'.",
+    "correct": "gracias",
+    "distractor": "perdón",
+    "explanation": "Gracias = thanks; perdón = sorry.",
+    "source_chapter": "11",
+    "source_title": "Useful Greetings & Farewells"
+  },
+  {
+    "sentence": "Reply to gracias: '_____ nada'.",
+    "correct": "De",
+    "distractor": "Por",
+    "explanation": "'De nada' literally 'of nothing' = you're welcome.",
+    "source_chapter": "11",
+    "source_title": "Useful Greetings & Farewells"
+  }
+]