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

client/tests/unit/test_content_db.gd

@@ -125,3 +125,8 @@ func test_blurb_content_carries_no_mechanics():
 		for banned in ["hit_die", "saves", "skill_pool", "skill_count", "armor", "talent"]:
 			assert_false(db.callings[id].has(banned),
 				"%s.json leaks the mechanic '%s' — that lives in Callings" % [id, banned])
+	for id in db.races:
+		for banned in ["save_bonus", "poison_save_bonus", "granted_skills", "flags",
+				"nightsight", "claws", "keen_scent"]:
+			assert_false(db.races[id].has(banned),
+				"%s.json leaks the mechanic '%s' — that lives in Races" % [id, banned])