Adds an opt-in 3D pet overlay pinned to the bottom-right. Gated behind
an experimental flag so three.js + GLB models stay out of the renderer
bundle for users who never enable the feature. Ships with four bundled
models plus user-uploaded custom GLBs via a pet:import IPC; a status-bar
segment provides model picker + hide toggle.
See docs/design/pet-overlay.md for the full design.
Co-authored-by: Orca <help@stably.ai>