Standardize bool vs boolean and NULL vs nullptr across the library

[jonfroehlich]

Problem

The library mixes two interchangeable spellings for the same concepts. Not a bug, but for teaching material that students read as a style exemplar, it should be consistent.

boolean vs bool

• boolean (Arduino typedef for uint8_t): FileUtils.hpp, ScrollingLineGraph.hpp, ScrollingLineGraphMultiValue.hpp, examples
• bool (standard C++): Button.hpp, Shape.hpp, MovingAverageFilter.hpp

NULL vs nullptr

• NULL is used throughout (e.g. MovingAverageFilter.hpp, GraphLine.hpp, FileUtils.hpp), even though the codebase already relies on C++11 (delegating constructors, = delete), so nullptr is available.

Proposed work

Pick one of each and apply repo-wide. Recommendation:

• Standardize on bool (standard C++; boolean is just an Arduino alias).
• Standardize on nullptr (type-safe, modern, and consistent with the C++11 features already in use).

Small, mechanical change; worth doing in one sweep so it reads cleanly for students.

[jonfroehlich]

Implemented in #8. Swept the whole library + examples to bool (over Arduino's boolean) and nullptr (over NULL); a leftover scan is clean. Will auto-close when #8 merges (held for hardware testing).