fix: use path slug instead of rkey for pckt URLs on atlas
pckt uses slug-based paths (e.g., /da-premier-post-9dmgs7z) not rkeys
in URLs. Include the path field from turbopuffer in atlas.json and
use it in atUriToUrl when available, matching the backend's
buildDocUrl logic.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>