Engram 0.15.13

Fixed

• Prepare the mascot asset once after a successful load and share pending work
  between concurrent requests. Give each project an independent recursive clone;
  failed loads remain retryable.
• Reuse fixed geometry for the arcane circle, conjure orb and holo screen while
  preserving independent project effect entities and material values.
• Forward the Show mascots setting to the scene. Hiding mascots disables their
  complete subtree and frame updates; showing them resumes retained entities
  and state.

These changes remove repeated preparation. Live measurements are still required
to establish startup, frame-rate or memory improvement.