Updated terminology around xorbs and cas objects.

[hoytak]

This PR updates the internal documentation of the xet package to reflect a recent pass to make "xorb" consistently refer to the data objects we use to store chunks, and "cas" consistently refer to the remote server / data store. Previously, both "xorb" and "cas" referred to the data object, which caused some confusion.

---

Note

Low Risk
Documentation-only changes that rename concepts/fields; low risk aside from potential reader confusion if any downstream code still uses the old names.

Overview
Standardizes Xet protocol documentation so "xorb" consistently refers to the on-disk chunk container object and "CAS" refers to the remote storage/service, removing prior mixed usage.

Updates the shard format spec and related upload/download/deduplication docs to rename CAS Info → Xorb Info (including struct/field names like cas_info_offset → xorb_info_offset, and CASReconstruction* → XorbReconstruction*) and adjusts a xet-core source link in xorb.md to the new xorb_object path.

^{Written by Cursor Bugbot for commit b90b3a1. This will update automatically on new commits. Configure here.}