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/articles-and-gender.json

@@ -0,0 +1,58 @@
+[
+  {
+    "sentence": "_____ libro está en la mesa.",
+    "correct": "El",
+    "distractor": "La",
+    "explanation": "libro ends in -o → masculine → el.",
+    "source_chapter": "04",
+    "source_title": "Articles"
+  },
+  {
+    "sentence": "Voy a _____ piscina.",
+    "correct": "la",
+    "distractor": "el",
+    "explanation": "piscina ends in -a → feminine → la.",
+    "source_chapter": "04",
+    "source_title": "Articles"
+  },
+  {
+    "sentence": "_____ problema es difícil.",
+    "correct": "El",
+    "distractor": "La",
+    "explanation": "Greek-origin -ma noun → masculine: el problema.",
+    "source_chapter": "04",
+    "source_title": "Articles"
+  },
+  {
+    "sentence": "Tengo _____ libro nuevo.",
+    "correct": "un",
+    "distractor": "uno",
+    "explanation": "Before a masculine noun use un, not uno.",
+    "source_chapter": "04",
+    "source_title": "Articles"
+  },
+  {
+    "sentence": "_____ ciudad es grande.",
+    "correct": "La",
+    "distractor": "El",
+    "explanation": "Nouns ending in -d are usually feminine.",
+    "source_chapter": "04",
+    "source_title": "Articles"
+  },
+  {
+    "sentence": "Bebo _____ agua fría.",
+    "correct": "el",
+    "distractor": "la",
+    "explanation": "agua takes el in the singular (stressed initial a-) even though it's feminine.",
+    "source_chapter": "04",
+    "source_title": "Articles"
+  },
+  {
+    "sentence": "Quiero ver _____ foto.",
+    "correct": "la",
+    "distractor": "el",
+    "explanation": "foto is short for fotografía → feminine.",
+    "source_chapter": "04",
+    "source_title": "Articles"
+  }
+]