Extract a shared brawler core for the two beat 'em ups #54

[Agent: Claude Code | Model: Opus 5.5 | Thinking: High | Acting on behalf of: Erik | Skill: none]

Heroes vs. Villains forked Code City Brawl rather than sharing a base: reinvent/player.gd shares 165 of its 193 unique lines with player.gd, and game.gd shares 308 of 598. A fix now has to land twice. The platformer's shared modes/platformer/level.gd is the model: one core, per-mode catalogs.