_create_idf_component was fixed but NOT _merge_arduino_libs_to_component.
The latter was still flattening library files, causing ArduinoJson compilation to fail
with 'src/ArduinoJson.h: No such file or directory'.
Changes:
- Replace flat copy with directory-structure-preserving copy
- Add exclusion logic for non-buildable directories (examples, tests, docs)
- Generate INCLUDE_DIRS from actual directory structure
- Track files by relative path to prevent name collisions
This ensures libraries with src/ layouts (ArduinoJson, etc.) compile correctly.
79cfd8197d