Fix scene variants and presence history

[agustin-lowpoly]

Fixes #35.

Summary:

• Generate singleplayer and deathmatch prepared scene JSONs and select the correct scene URL at runtime.
• Cover gameplay/input gaps around zero-travel buttons, movement, and damageable brush targeting.
• Add multiplayer presence history buckets with focused tests.

Tests:

• node --test test/prepare/sceneVariants.test.mjs test/runtime/entities.test.mjs
• node --test test/multiplayer/presence.test.mjs
• pnpm build