Remove avatar assets without redistribution provenance

[a-baran-orhan]

Summary

Follow-up to posecode-dev#124 that keeps the avatar selector and renderer hot-swap API while removing bundled character files whose redistribution provenance has not been established.

• remove avatar2.glb and avatar3.glb
• remove their built-in playground and embed URL mappings
• preserve avatar avatar2 / avatar avatar3 parsing and host-provided characterUrls
• use the procedural figure when those selectors are unmapped
• document the fallback and redistribution requirement
• test that the embed defaults do not publish avatar2 / avatar3 URLs

This avoids blocking the extensible API on asset licensing. Licensed assets can be added later with source and redistribution terms documented.

Verification

• npm test — 47 files, 463 tests passed
• npm run build:packages
• npm run typecheck
• npm run build
• npm run check:packages
• git diff --check