[codex] Expose S3TC compressed texture extensions

[dsafdsaf132]

Summary

• Expose WEBGL_compressed_texture_s3tc and WEBGL_compressed_texture_s3tc_srgb through the WebGL extension registry.
• Add S3TC extension constants to the native addon and TypeScript surface.
• Document backend-dependent exposure and add WebGL2 smoke coverage for constants, supported compressed formats, and 4x4 compressed uploads when available.

Validation

• npm run build
• npx --yes node-gyp rebuild
• npm run test:webgl2
• npm test
• git diff --check