Initial commit: ESP32 NextPM sensor reader

Educational version with comprehensive comments for students.
- Reads PM1.0, PM2.5, and PM10 concentrations from NextPM sensor
- Uses UART serial communication with checksum validation
- Non-blocking timing with millis()
- Removed particle counts, kept only mass concentrations
- Extensively commented to help students understand serial protocols

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

test/README

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+
+This directory is intended for PlatformIO Test Runner and project tests.
+
+Unit Testing is a software testing method by which individual units of
+source code, sets of one or more MCU program modules together with associated
+control data, usage procedures, and operating procedures, are tested to
+determine whether they are fit for use. Unit testing finds problems early
+in the development cycle.
+
+More information about PlatformIO Unit Testing:
+- https://docs.platformio.org/en/latest/advanced/unit-testing/index.html