Add runtime atlas follow-up polish

[agustin-littlehat]

Summary

• replay the runtime atlas follow-up commits onto current main after the previous main-missing-runtime-atlas branch was already merged
• add source-aligned fall damage with grounded-descent and water-block protections
• keep impact particles, weapon render-bundle presentation, and prewarm/runtime polish together on a fresh PR branch

Validation

• pnpm test
• pnpm build