Add a thin dual-SSD1683 driver (src/epd.rs, ported from GxEPD2's
GxEPD2_579_GDEY0579T93) for the 792x272 e-paper panel: reset/init,
RAM addressing, the split-and-mirror full-frame blit across the
master/slave seam at x=396, and full-refresh waveform. Frame
implements embedded-graphics' DrawTarget.
Replace the Spike 1 blink harness in main.rs with the Spike 2 test:
SPI at 4 MHz over SCK 12 / DIN 11 / CS 7 / DC 6 / RST 5 / BUSY 4,
alternating normal/inverted frames with a seam-straddling circle,
"Typoena", and the build tag. Verified on hardware 2026-07-04.
