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>

world/locations/kingdom-of-corvel.md

@@ -21,7 +21,7 @@ Drenwick, Phelan's home base, is a provincial river-junction city in the south o
 | Duchy | Position | Seat | Ruler | Character |
 |---|---|---|---|---|
 | **Thorngate** | North | Thorngate | Duchess Pamira Varnesse | Old blood, widowed, estate under quiet financial strain. Uses her jurisdictional standing to deflect Compact delegations. |
-| **Aldermere** | North (peer to Thorngate) | Aldermere | Duke Garrett Holven | Moderate, horse-breeder nobility. Married into the seat. The duchy Charlette Fields calls "the northern provinces." |
+| **Aldermere** | North (peer to Thorngate) | Aldermere | Duke Garrett Holven | Moderate, horse-breeder nobility. Married into the seat. For a decade he has been quietly acquiring distressed noble debt across the north — including, recently, an unresolved obligation left behind by Duchess Pamira's late husband. He has not moved on it. He could. The duchy Charlette Fields calls "the northern provinces." |
 | **The Crownhold** | Central-north | Varenhold (capital) | King Halvar Rethen III *(held directly)* | Bureaucratic crown rule. Late 50s, widowed, competent-but-tired. One heir. |
 | **Wenlow** | West | Wenlow | Duchess Isolde Merrenwood | Agricultural heartland — grain, livestock, wine. Rich, pragmatic, deeply tied to the merchant guilds. |
 | **Vethmarch** | East / frontier | Arvale Keep | Duke Cason Arvale | Young, ex-military, inherited recently. Holds the mountain frontier, Vethek Pass, and the Vethani Crypts. Pathfinder-adjacent. |