One report per phase or feature branch, named after the branch:
phase-0-test-infra.md, phase-3b-domain-core.md, and so on.
Required sections and rules are in docs/WORKFLOW.md §6. The short version:
the git history records what changed; the report records what happened —
deviations from the phase document, scope changes, real test output, and
anything learned about the hardware.
A report that only says "built what was asked, all tests pass" is either wrong or not looking hard enough.