Both promotion_config() and build_registry() now pull raw bytes from
content::get(Promotions) instead of a crate-local include_str! const. The
embedded fallback moved to the central ContentRegistry, so the Rail-2 gate gains
a registry_owned flag: Check A verifies mc-core/src/content.rs embeds the JSON,
while the XP_THRESHOLDS/HEAL_ON_PROMOTE_FRACTION tombstones still guard
mc-combat. Gate passes; mc-combat/mc-turn/mc-player-api tests green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>