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/stressed-possessives.json

@@ -0,0 +1,50 @@
+[
+  {
+    "sentence": "Estos libros son _____ (mine).",
+    "correct": "míos",
+    "distractor": "mío",
+    "explanation": "Plural masculine noun (libros) → míos.",
+    "source_chapter": "32",
+    "source_title": "Stressed Possessive Adjectives"
+  },
+  {
+    "sentence": "La casa es _____ (ours).",
+    "correct": "nuestra",
+    "distractor": "nuestro",
+    "explanation": "Casa is feminine singular → nuestra.",
+    "source_chapter": "32",
+    "source_title": "Stressed Possessive Adjectives"
+  },
+  {
+    "sentence": "Una amiga _____ (of mine) llegó ayer.",
+    "correct": "mía",
+    "distractor": "mío",
+    "explanation": "Amiga is feminine → mía.",
+    "source_chapter": "32",
+    "source_title": "Stressed Possessive Adjectives"
+  },
+  {
+    "sentence": "Los problemas son _____ (theirs).",
+    "correct": "suyos",
+    "distractor": "suya",
+    "explanation": "Problemas is masc. plural → suyos.",
+    "source_chapter": "32",
+    "source_title": "Stressed Possessive Adjectives"
+  },
+  {
+    "sentence": "El coche _____ (of yours, tú) es nuevo.",
+    "correct": "tuyo",
+    "distractor": "tuya",
+    "explanation": "Coche is masc. sing. → tuyo.",
+    "source_chapter": "32",
+    "source_title": "Stressed Possessive Adjectives"
+  },
+  {
+    "sentence": "Las ideas _____ (yours, vosotros) son buenas.",
+    "correct": "vuestras",
+    "distractor": "vuestros",
+    "explanation": "Ideas is fem. plural → vuestras.",
+    "source_chapter": "32",
+    "source_title": "Stressed Possessive Adjectives"
+  }
+]