fix(creation-model): migrate stray class_id/allowed_classes copies

The canon-log schema and NewGame code already migrated to race_id/
calling_id + allowed_callings, but two copies of the roster outside
that schema were missed, and no parity test guards them:

- docs/canon-log.md (the cross-boundary contract doc) still documented
  the OLD player shape and allowed_classes. Anyone building a player
  block from the doc would get a 422.
- .claude/skills/world-building's schema reference still emitted
  allowed_classes with two dead calling ids (assassin, priest). Author
  a new origin with that skill and it fails origin.schema.json
  (additionalProperties: false, allowed_callings required) AND, if that
  ever loosened, silently allows zero callings at runtime.

Also:
- add schema tests rejecting a dead class_id field and an unknown
  calling_id (priest)
- guard NewGame._validate's container types (spend/skills) so a
  malformed JSON round-trip (null spend, string skills) produces a
  front-loaded error list instead of a GDScript runtime crash
- extend the no-mechanics-in-content test to db.races, not just
  db.callings
- rewrite the roadmap's M4 bullet: the two contract migrations landed;
  only the creation screen and title->creation->shell flow remain

client: 250/250. api: 74 passed, 2 skipped. content_build --check: clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QYa9u7Kdxv5gX4AnwWexy8

.claude/skills/world-building/references/schema.md

@@ -252,7 +252,7 @@ construction fails loudly.
   "disposition_overrides": { "brannoc_thane": 40, "cadwyn_vell": 15 },
   "inventory_grants": [{ "item_id": "worn_shortsword", "qty": 1 }],
   "start_quest_id": "find_the_ledger",
-  "build_constraints": { "allowed_classes": ["sellsword","assassin","priest"], "luck_modifier": 0 }
+  "build_constraints": { "allowed_callings": ["sellsword","reaver","cutpurse","trapper","hedge_mage","bonesetter","bloodsworn"], "luck_modifier": 0 }
 }
 ```