Add example sketches for classes that lack them (also gives them CI coverage)

[jonfroehlich]

Problem

Several public classes ship with no example sketch, even though Arduino surfaces examples prominently (File → Examples → Makeability Lab) and they're the primary way students discover a class. The bundled examples only cover shapes/joystick/games.

Classes with no example:

• Button
• MovingAverageFilter
• ColorName
• FileUtils
• ScrollingLineGraph
• ScrollingLineGraphMultiValue

As a side effect, these classes are never compiled by CI (the Compile Examples workflow only builds examples/), so regressions in them won't be caught.

Proposed work

Add a minimal example sketch per class. This also gives each one free CI coverage. Good candidates:

• Button/ButtonBasic
• MovingAverageFilter/SmoothAnalogInput
• ColorName/ClosestColorMatch
• FileUtils/ListSdFiles
• ScrollingLineGraph/GraphAnalogInput
• ScrollingLineGraphMultiValue/GraphTwoInputs

Notes

• The README already has usable code snippets for most of these — they can seed the examples.
• Several existing arduino-repo sketches already exercise ScrollingLineGraphMultiValue and can be adapted.

[jonfroehlich]

Implemented in #8. Added example sketches for every previously-uncovered class: ButtonBasic, SmoothAnalogInput, ClosestColorMatch, ListSdFiles, ScrollingLineGraphA0, and ScrollingLineGraphTwoInputs (plus SimpleLineGraphA0 for the new class). All compile on arduino:avr:uno and CI now installs the SD library for the FileUtils example. Will auto-close when #8 merges (held for hardware testing).