add PLATFORMIO_CORE_DIR - #91

No actionable comments were generated in the recent review. 🎉

Merge Risk: ⚪ Minimal · up to 30d96

The build configuration change confines the PlatformIO core directory to the package build directory without identified merge-blocking impact.

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

_{A rabbit hops through builds so neat
The core now rests where paths meet
Inside the package tree
A tidy dot-directory
Makes PlatformIO complete}

_{Comment @coderabbitai help to get the list of available commands.}