Add off-device unit tests for pure-logic classes

[jonfroehlich]

Now that compile CI exists (#1), the higher-value testing layer is unit-testing the classes whose logic is hardware-independent. This naturally belongs in this repo rather than the arduino repo.

Candidates (no hardware needed)

• MovingAverageFilter — sliding-window average, isWindowFull() behavior.
• Shape / Circle / Rectangle / Ball — bounding-box & radial collision math, containment, boundary clamping.
• ColorName — nearest-color (MSE) matching.

Decisions to make

• Framework: host build with GoogleTest + ArduinoFake to stub Arduino.h, vs. AUnit on an emulator. Recommend GoogleTest + ArduinoFake for fast, hardware-free ubuntu-latest runs.
• Layout: a test/ dir + CMake, wired into a new CI job.

Acceptance

• Tests run on push/PR in GitHub Actions (separate job from the compile workflow).
• Coverage for the three classes above.

Tracking follow-up from #1.

[jonfroehlich]

Looks like ArduinoFake hasn't been updated for nine months with 29 open Issues.