## Why
Built-in pet downloads used a standalone blocking client, so CDN redirects did
not use Codex's configured route handling.
## What changed
- Download pet spritesheets asynchronously with a reusable
`RouteAwareClientPool`.
- Keep cache validation, asset installation, and pet decoding off the async
runtime while sharing one asset-and-load path across startup, previews, and
selection.
- Preserve the download size limit while consuming streamed response chunks,
and remove the TUI's direct `reqwest` dependency.
## Testing
Add coverage for oversized response chunks, cached built-in assets, pet loads
without an existing Tokio runtime, and stale preview and selection completions.
GitOrigin-RevId: 725e76f3ddb7e84b7212769892f28dc0add15741