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/22-stem-changing-verbs.md

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+# 22. Stem-Changing Verbs
+
+> Source: [video](https://youtube.com/watch?v=YHDZSHCt1DE&t=5579s)
+
+Stem-changing verbs modify their internal vowel in conjugation to "sound better." Spanish has four categories: e→ie, e→i, o→ue, and u→ue. The change occurs in all forms EXCEPT nosotros and vosotros (the "boot" pattern).
+
+## Key Rules
+- The stem change applies to yo, tú, él/ella/usted, ellos/ellas/ustedes — NOT nosotros / vosotros.
+- Four categories: **e → ie**, **e → i**, **o → ue**, **u → ue** (only `jugar`).
+- Endings stay regular; only the stem vowel changes.
+- If you see a verb conjugated with `ie`, `i`, or `ue` in the stem, it's likely a stem-changer.
+
+## Conjugation / Pattern Tables
+
+### empezar (e → ie) — to start
+| Pronoun | Form |
+|---|---|
+| yo | empiezo |
+| tú | empiezas |
+| él/ella | empieza |
+| nosotros | empezamos |
+| vosotros | empezáis |
+| ellos | empiezan |
+
+### decir (e → i) — to say (also irregular yo: digo)
+| Pronoun | Form |
+|---|---|
+| yo | digo |
+| tú | dices |
+| él/ella | dice |
+| nosotros | decimos |
+| vosotros | decís |
+| ellos | dicen |
+
+### recordar (o → ue) — to remember
+| Pronoun | Form |
+|---|---|
+| yo | recuerdo |
+| tú | recuerdas |
+| él/ella | recuerda |
+| nosotros | recordamos |
+| vosotros | recordáis |
+| ellos | recuerdan |
+
+### jugar (u → ue) — to play
+| Pronoun | Form |
+|---|---|
+| yo | juego |
+| tú | juegas |
+| él/ella | juega |
+| nosotros | jugamos |
+| vosotros | jugáis |
+| ellos | juegan |
+
+## Examples
+| Spanish | English |
+|---|---|
+| Yo empiezo el trabajo a las ocho. | I start work at eight. |
+| Tú dices la verdad. | You tell the truth. |
+| Él recuerda mi nombre. | He remembers my name. |
+| Nosotros jugamos al fútbol. | We play soccer. |
+| Ellos pueden venir mañana. | They can come tomorrow. |
+
+## Notes & Gotchas
+- `jugo` on its own = "juice"; the verb stem-changes to `juego` to avoid confusion. `videojuego` = video game.
+- `tener` and `poder` are stem-changers you've already met (tengo/tienes…, puedo/puedes…).
+- Nosotros/vosotros stay regular — that's the "boot" shape on a conjugation chart.