chore(validation): seed reproducible Crosscheck defect

[ruby-dlee]

Intent

This is a disposable validation PR for the Release 2B2 Crosscheck checkpoint. It deliberately introduces a small, helper-reproducible defect: bounded_slice returns every value when its limit is zero, contrary to its documented contract.

The adjacent executable regression demonstrates the defect and intentionally exits nonzero. This PR must not merge. It will be closed after Crosscheck admits the defect as a durable finding through the new incremental Pi tools and isolated proof compartments.

Reproduction

python3 validation/crosscheck_v2/test_bounded_slice.py

Expected validation result: nonzero exit from the deliberate assertion failure.

[ruby-dlee]

Validation checkpoint parked after the official wrapper completed the model pass but waited more than 31 minutes for shared proof-compartment capacity. The run was interrupted, cleanup unwound, and no verdict was admitted. Closing this disposable validation PR without merge.