Add a hardware-free pytest suite and a minimal CI workflow

[egeboy35]

This PR adds a small automated test suite and CI setup for the pure-DSP parts of the repository.

Most existing test_*.py scripts cannot run unattended. Measured in a plain numpy/scipy/matplotlib environment, 22 of the 33 fail at pytest collection: 7 need pyadi-iio, 1 needs torch, 1 cv2, 1 PyQt6, and 12 fail only because the flat-layout modules (sdr_video_comm, myadiclass and friends) are not importable from the repository root. So regressions in the math-only paths were easy to miss.

What's included

• tests/ — 22 pytest tests, all hardware-free and deterministic (seeded RNGs, first-principles golden values): OFDM/OTFS modulate-demodulate loopbacks and flat-channel equalization, convolutional FEC (Viterbi) round trip and error correction, Schmidl-Cox preamble sync on a delayed/rotated/noisy signal, the RF packet protocol (CRC32 framing, chunking) and QPSK Gray mapping, the 802.11a LTF autocorrelation, the OFDM FFT-shift convention, and AIRadarLib (radar parameter derivation, chirp/tone spectra, CA-CFAR detection, target masks, noise SNR calibration).
• Torch-optional — the 5G LDPC round trip and the CFAR tests gate on pytest.importorskip("torch") and skip cleanly when torch is absent; the LDPC test is marked slow (-m "not slow" deselects it). Without torch the suite reports 19 passed and 2 skipped of the 22 (the two skips cover 3 test functions, since one module skips at collection).
• tests/conftest.py — sets MPLBACKEND=Agg before any matplotlib import, reconfigures stdout to UTF-8 (some modules print Unicode that breaks Windows cp125x consoles), and derives the sdradi/pluto_test/AIRadar sys.path entries from __file__.
• .github/workflows/ci.yml — a lint job and a test job on Python 3.11 with the CPU-only torch wheel. Three deliberate choices: the compileall syntax guard runs before the linter so a style finding cannot mask a real syntax regression; ruff is pinned so a future release cannot turn CI red without a code change; and push builds are limited to main so pull requests are not built twice.
• pytest.ini / requirements-dev.txt — restrict collection to tests/ (the hardware and flat-layout scripts elsewhere are never collected) and list the test dependencies.

Everything is additive: no existing file is modified.

Verified locally with ruff 0.16.3 (clean over tests/), compileall (exit 0 over all six packages), and pytest. I also checked the gates against my other open PRs by merging them together locally: #1..#8 merge without conflict and the lint and test gates both pass on the merged tree, so this workflow should not paint the other PRs red.

If a workflow file under .github/ from an outside contributor is not something you want to take on, the tests/ directory stands on its own — happy to drop the CI half.

🤖 Generated with Claude Code