Per keystroke, diff the new frame against the last shown one to find the
changed row band and partial-refresh only those rows instead of all 272.
E-paper update time scales with the gate rows driven, so a single edited
text line refreshes far faster. Windowed in Y only (full width, both
controllers), so the seam/mirroring logic is unchanged. Also drop the
periodic full refresh to every 64 updates — the panel stays visually
clean, so it's now mainly for longevity.
display_frame_partial writes the new image to bank 0x24, runs GxEPD2's
partial waveform (_Update_Part: 0x3C/0x80, 0x21/0x00,0x10, 0x22/0xFF,
0x20), then syncs bank 0x26 so the next partial has a correct baseline.
Like GxEPD2 for this dual-controller panel, the update covers the full
panel with the partial waveform — no windowing, since the waveform time
dominates. Much faster than a full refresh and without the flashing.
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.
