License header consistency

Some but not all files have the Apache license boilerplate at top; should we ensure we consistently have this language on all files and perhaps find a way to ensure that it's present automatically?

No files, tests, or entry points are named. Start by inventorying repository files with and without the Apache boilerplate, then determine how consistency could be checked automatically; done means the desired policy is defined, existing files follow it, and any enforcement mechanism is identified.

Written by the indexing model from the issue text.