Re-ran the spike on the dedicated SPI3 wiring (CLK 14 / MOSI 15) with
the same clean mount + atomic round-trip result. Flip the docs from
"pending a re-run" to verified.
Document the shared-bus arbitration decision: rather than rework the
proven EPD SPI layer and add a cross-thread mutex on the save path, the
SD moves to a dedicated SPI3. Update the boot sequence, risk table,
Spike 3 postmortem follow-up, hardware overview, and firmware README.
Bare-folder links (e.g. docs/postmortems/) only resolve on GitHub's
auto-render; retarget them to the folder's README.md so they open a
real file in any markdown viewer.
