[dasc] Checkbox default not persisted, validation error targets wrong array item

Observed the following issues:

1. Unchecked checkboxes appeared correct in the UI but `false` was not persisted when the value was missing.

2. Validation error might apply to a different item in the array (happens when the array contains empty items, which are pruned from the JSON before validation).

No files, tests, or entry points are named. Start by locating the DASC checkbox persistence and array-validation paths, then reproduce both cases: a missing unchecked value should persist as false, and validation errors should remain attached to the correct array item after empty items are pruned.

Written by the indexing model from the issue text.