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/notes/12-the-verb-poder.md

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+# 12. The Verb "Poder"
+
+> Source: [A Complete Guide To Every Fundamental In Spanish](https://youtube.com/watch?v=YHDZSHCt1DE&t=2633s)
+
+`Poder` means "to be able to" / "can." It's a stem-changing verb (o → ue) and one of the most useful verbs in Spanish because it pairs with any infinitive to express possibility.
+
+## Key Rules
+- `poder` is a stem-changing verb: **o → ue** in the singular and 3rd-person plural forms (the "boot").
+- The `nosotros` and `vosotros` forms keep the original `o` (`podemos`, `podéis`).
+- Endings still follow the regular `-er` pattern.
+- Use `poder + infinitive` to express ability or possibility: *Yo puedo hablar español.*
+
+## Conjugation: poder (present indicative)
+
+| Pronoun | Form |
+|---------|------|
+| yo | puedo |
+| tú | puedes |
+| él / ella / usted | puede |
+| nosotros | podemos |
+| vosotros | podéis |
+| ellos / ellas / ustedes | pueden |
+
+The "boot" pattern: `puedo / puedes / puede / pueden` change the stem; `podemos / podéis` stay regular.
+
+## Examples
+| Spanish | English |
+|---------|---------|
+| Yo puedo hablar español. | I can speak Spanish. |
+| Tú puedes aprender inglés. | You can learn English. |
+| Él puede ayudar. | He can help. |
+| Nosotros podemos ir. | We can go. |
+| Ellos pueden llamar por teléfono. | They can call by phone. |
+| ¿Puedes ayudarme? | Can you help me? |
+| No puedo dormir. | I can't sleep. |
+
+## Notes & Gotchas
+- The stem change applies **only** in the "boot" forms — never in `nosotros`/`vosotros`.
+- Always follow `poder` with an **infinitive** (unchanged verb): `puedo hablar`, not `puedo hablo`.
+- `poder` doesn't take a preposition before the infinitive (no `a`, no `que`).
+- Other common stem-changing verbs follow the same boot pattern (e.g., `dormir` o → ue, `querer` e → ie).