engine is captured by src/highlevel/native.js and deleted from globalThis
after $ is installed; the module loader refuses r2d/* internals to game
code. Agent eval sees engine only for the duration of the call. Adds
$.startScene, $.gfx.white, $.time.perfNow; demos, game, web scene and
fixtures migrated to $.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>