Adds three cached Apptainer images covering all 6,710 published CrossCodeEval tasks.
The images are clean builds of the existing task environments. All three representative tasks passed build/start checks. Task contents, validation results and datasource READMEs are unchanged.
The task Parquets now include image references pinned to a Hugging Face commit. The validation launcher can download matching images automatically and verify their checksums instead of rebuilding them.
Image coverage and checksums are recorded in `images/crosscodeeval.json`.
Explain datasource downloads and automatic image caching
