docs(adr): record ADR-012 — SD on its own SPI3 host

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.

docs/postmortems/2026-07-05-spike3-sd-cmd59.md

@@ -190,8 +190,9 @@ it is explicitly **not recommended** and not applied.
       (2026-07-11).
 - [x] Enable PSRAM (done, `CONFIG_SPIRAM`) and build Spike 7 (git push) — both
       complete; see [Spike 7 postmortem](2026-07-05-spike7-gix-https-push.md).
-- [ ] **Still open:** settle the **shared-bus arbitration** decision (EPD lock
-      vs. SPI3 for SD) before wiring persistence into `main.rs`.
+- [x] Settle the **shared-bus arbitration** decision → **SD on its own SPI3**
+      (ADR-012, 2026-07-11); spike rewired to SPI3 (SCK 14 / MOSI 15), pending a
+      bench re-run on the new wiring.
 - [ ] **Still open:** implement the FatFS atomic-save (unlink-then-rename +
       `*.tmp` boot-recovery) in the real `persistence` module.