- Add board.rs to all 8 drivers: constants, type aliases, init functions,
and HAL-specific helpers with full docstrings and pub(crate) visibility
- Slim main.rs to boilerplate + main() only, zero helper functions
- Fix i2c.rs: add file header, full docstrings on all functions
- Fix lcd1602.rs: add file header, full docstrings on all functions
- Fix lib.rs headers for 0x07 and 0x08
- All 8 drivers build and all 75 tests pass
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