Rename `_CHALKS` to `_ARTIFACT_INFO`

The `_CHALKS` key contains metadata about artifacts, some of which are part of the chalkmarks (those without `_`) and some of which are part of the per-artifact runtime info. They do _no_ correspond to chalkmarks. We should probably rename them to prevent any confusion on behalf of the users

Search the repository for `_CHALKS` and inspect every reference, including tests and user-facing documentation. Rename the key consistently to `_ARTIFACT_INFO`, preserve the existing artifact metadata behavior, and run the affected test suite to confirm all references still work.

Written by the indexing model from the issue text.