Add PWM test report (pwm.md) with 4 oscilloscope measurements on GPIO48. Restructure root README (fix section numbering, env var paths, BSP compliance, community plan). Update board-level README with test data and reproduce checklist. Embed all 4 oscilloscope screenshots directly in both READMEs.

Ultraworked with Sisyphus (https://github.com/code-yeongyu/oh-my-openagent

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Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>