book3: update worldbuilding for ruin backstory revision

- kingdom-of-corvel.md: Holven as debt-acquirer pattern
- pip.md: rewrite as true dragon from Mine 1 earth-magic mutation,
  add growth mechanics section
- supporting-cast.md: Sable as returning forensics scout,
  add Holven row

Ref: docs/superpowers/specs/2026-04-19-book3-ruin-backstory-revision-design.md

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

chapters/book3/CLAUDE.md

@@ -67,7 +67,7 @@ Pure Phelan first-person, past tense, throughout. Kimbra's subplot is processed
 | Brennan Toor | Off-page. Cairns infrastructure | `characters/brennan-toor.md` |
 | **Duchess Pamira** (new) | Client, Devod's love interest | `characters/duchess-pamira.md` |
 | **Kimbra** (new) | Phelan's mother, reconnection subplot | `characters/kimbra.md` (to create) |
-| **Pip** (new) | Pixie dragon, Mere-bonded, tactical magic detector | `characters/pip.md` (to create) |
+| **Pip** (new) | True dragon (Mine 1 lineage), Mere-bonded, tactical magic detector | `characters/pip.md` (to create) |
 | **Sable** (new, minor) | Ledger's field operative | `characters/supporting-cast.md` |
 | **Seraphel "Sera"** (new, in utero) | Phelan/Mere's unborn daughter | `characters/supporting-cast.md` |