[NT7S]

Adds Test Lab's Si5351 regression harness under tests/regression and a GitHub Actions workflow (.github/workflows/regression.yml) that runs it on every pull request and on pushes to non-master branches.

Report-only. This is not a required check and never gates merges. No branch protection changes. Existing main.yml (lint/compile) and size-deltas.yml are untouched. No license changes.

Approved by Jason Milldrum (NT7S) on 2026-10-30.

What it runs

tests/regression/run_v231_checks.sh --src $GITHUB_WORKSPACE builds src/si5351.cpp natively with g++ against host Arduino/Wire stubs, then checks:

• 776 must-not-break register/state locks, 27 original golden cases plus 23 newer ones, the issue Lost the ability to connect the Si5351 via the Etherkit library to an I2C channel other than channel 0. #106 TwoWire test, and the Python AN619/datasheet oracle suites
• 25 v2.3.1 acceptance tests. On master these are expected failures (XFAIL) because they cover bugs v2.3.1 fixes. They count as passing in default mode. Run --require-fixed on the v2.3.1 branch to make them hard failures.

A summary is written to the job summary, and logs are uploaded as an artifact.

Expected status on master

Green: PASS 7, XFAIL 25 (v2.3.1 bugs), INFO 3, 0 FAIL. Latest run on the PR: https://github.com/etherkit/Si5351Arduino/actions/runs/41141012656 (green, same counts).

Harness by Test Lab, vendored unchanged from its working copy (ci/, without generated runs/).

Notes

• tests/regression/sketches/Ctor106/Ctor106.ino is stored as Ctor106.ino.txt, because arduino-lint in main.yml rejects sketches outside examples/. run_v231_checks.sh copies it back to .ino when it builds with --cores. That's the only change from Test Lab's copy.
• The existing CI workflow's compile jobs fail on this PR, and that isn't caused by this change. ubuntu-latest is now Ubuntu 26.04, and arduino/compile-sketches@v1 asks for Python 3.12.4, which isn't available for 26.04. Lint passes. main.yml is left untouched as asked, so this needs its own fix, such as pinning runs-on: ubuntu-24.04.