Add a faster full-public mirror for China and restricted networks

[Dsadd4]

Remote validation confirmed the full-public pack works, but GitHub release object storage can be slow from some cloud networks. A faster mirror would make one-command setup more reliable for global users.

Scope:

• Add a mirror manifest beside the Hugging Face and GitHub release manifests.
• Keep checksums identical to the existing full-public archives.
• Document mirror priority in docs/FULL_KB_DISTRIBUTION.md.
• Make agentfiguregallery setup report clear download speed and resume status.

Validation:

• agentfiguregallery setup --pack full-public --manifest <mirror-manifest> completes from a fresh remote server.
• Checksum verification passes.
• No private paths or private repositories are introduced.