fix(epd2in7b): correct V1 white and black LUT mapping

[tkj666]

Summary

• match Waveshare's 2.7-inch B V1 reference by loading LUT_BB into register 0x23 and LUT_WB into register 0x24
• add a regression test that observes the public Epd2in7b::new initialization sequence at the SPI/DC boundary

Problem

The existing V1 driver assigns the two arrays according to their transition-style names. In B/W/Red mode, however, Waveshare's reference implementation intentionally loads them in the opposite-looking order. The current assignment swaps the final white and black drive waveforms, so a correct image can appear during the refresh phases and then settle with inverted black/white colors.

Reference: https://raw.githubusercontent.com/waveshareteam/e-Paper/master/RaspberryPi_JetsonNano/c/lib/e-Paper/EPD_2in7b.c

The framebuffer-byte complement remains unchanged because it already matches the reference driver.

Verification

• regression test failed before the mapping change and passes afterward
• cargo test --lib — 55 passed
• cargo check --lib
• RUSTDOCFLAGS=-Dwarnings cargo doc --no-deps
• cargo clippy --lib --tests -- -D warnings -A clippy::new_ret_no_self -A clippy::doc_overindented_list_items
• cargo fmt --all -- --check