Code-quality polish: dead code, debug leftovers, doc stubs, minor robustness

[jonfroehlich]

Problem

A cluster of small polish items surfaced during a code-quality review. None is urgent, but collectively they affect how clean the library reads as teaching material. Tracking them together.

Code hygiene

• Dead member: GraphLine::_sampleTotal is declared and honestly commented "unused currently" (GraphLine.hpp). Remove it rather than ship documented dead state.
• Commented-out debug Serial.printlns left in source: FileUtils.hpp (the "ends with"/"does NOT end with" lines) and Shape.hpp ("We are in overlaps shape!"). A published library should be clean of these.
• Empty Doxygen stubs: several @param/@return lines have no text and read as auto-generated boilerplate (e.g. Shape::distance params; Circle::overlaps @return true/@return false; many graph getters). Fill or drop them.

Robustness

• MovingAverageFilter has no guard on windowSize. Doc says "must be > 0" but new int[windowSize] and getAverage()'s / _windowSize will misbehave for 0/negative. Add a guard or at least an explicit "undefined if <= 0" note.
• ScrollingLineGraph buffer sizing _bufferSize = _widthGraph - _xGraph; has no guard against width < xGraph (negative allocation). Low real-world risk.

Teaching-code consistency

• Shape::distance uses pow(dx, 2) — on AVR that's a slow float call; dx*dx + dy*dy is faster, exact, and a nice teaching point. Called per-frame in circle collisions.
• CollisionTest.ino never delete[]s _targetShapes. Harmless on a microcontroller (lives for program lifetime) but, in code that explicitly demonstrates new, model the cleanup rule taught elsewhere.
• Example footers (Pong.ino, CollisionTest.ino, others) still use the old @jonfroehlich style, not the shared footer the .hpp headers adopted — finishes the footer-alignment work.

Repo niceties (optional)

• Add CHANGELOG.md (releases are now versioned — 0.2.0).
• Add CONTRIBUTING.md.

[jonfroehlich]

Mostly addressed in #8:

• Remove dead GraphLine::_sampleTotal
• Remove commented-out debug Serial.printlns (FileUtils, Shape)
• Fill empty Doxygen stubs (Shape::distance params, Circle::overlaps returns)
• MovingAverageFilter window-size guard
• ScrollingLineGraph buffer-size guard
• Shape::distance uses multiplication instead of pow(x, 2)
• CollisionTest lifetime/cleanup note for the new'd shapes
• Example footers aligned to the shared convention
• Add CHANGELOG.md
• Add CONTRIBUTING.md ← only remaining item

Leaving this open to track CONTRIBUTING.md after #8 merges.