docs(sd): record Spike 3 hardware verification and findings

Mark Spike 3 resolved (genuine 32 GB SDHC mounts, round-trips clean on the
shared SPI2 bus; the 133 GB SDXC's CMD59 rejection was the sole fault).
Capture the FatFS f_rename caveat and the ≤32 GB card-compatibility note in
ADR-007, the resolution in the postmortem, and the verified writeup in the
firmware README.

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

@@ -1,8 +1,10 @@
 # Spike 3 (SD) — blocked on a card that rejects CMD59 (SPI-mode CRC)
 
 > Date: 2026-07-05 · Build at time of failure: `07-05 16:07Z @f77f669-dirty`
-> Status: **paused, not failed** — bench card is incompatible; awaiting a
-> compliant microSD. Wiring and firmware are proven good.
+> Status: **RESOLVED 2026-07-11** — a genuine 32 GB SDHC card mounts, negotiates
+> 10 MHz, and round-trips cleanly on the shared SPI2 bus. Wiring and firmware
+> were always good; the 133 GB SDXC card's CMD59 rejection was the sole fault.
+> See [Resolution](#resolution-2026-07-11) below.
 >
 > Context: Spike 3 in
 > [`../v0.1-mvp-technical.md`](../v0.1-mvp-technical.md#hardware-bring-up-order),
@@ -10,6 +12,39 @@
 > firmware notes [`../../firmware/README.md`](../../firmware/README.md).
 > Spike program: [`../../firmware/src/bin/sd_fat.rs`](../../firmware/src/bin/sd_fat.rs).
 
+## Resolution (2026-07-11)
+
+Re-ran `sd_fat` with a genuine 32 GB SDHC card (build `07-11 08:53Z
+@195311a-dirty`). Everything the paused write predicted held:
+
+- `sdspi_host: data CRC set=1` — **CMD59 now succeeds**. Init runs clean through
+  card identification (`sdmmc_card_init: card type is SD`).
+- `card mounted at /sd — max 10000 kHz, negotiated 10000 kHz`.
+- `FAT usage — 29806 MiB total, 29802 MiB free` (~29.8 GiB of the 32 GB card).
+- `round-trip OK — 92 bytes: create …tmp → fsync → rename → read back identical`.
+
+Wiring, pull-ups, bus sharing, and the hand-rolled FFI mount path all confirmed
+correct — the only variable that changed was the card, exactly as diagnosed. The
+`cmd=52` / `cmd=5` "command not supported" lines still print and are still
+normal (SDIO probes a memory card ignores).
+
+### New finding: FatFS `f_rename` does not overwrite (atomic-save is not POSIX)
+
+The first run created `/sd/spike3.md`; the **second** run failed at `rename tmp
+-> final` with `File exists (os error 17)`. Root cause is a FatFS semantics
+difference, not a fault: **`f_rename` returns `FR_EXIST` when the destination
+already exists** — unlike POSIX `rename(2)`, which atomically replaces it. So the
+textbook write-`*.tmp` → rename-over-target idiom from ADR-007 does **not** work
+as-is on FAT; the destination must be unlinked first.
+
+That unlink opens a crash window: between `f_unlink(target)` and `f_rename`, the
+target is momentarily gone while `*.tmp` holds the complete, fsync'd new content.
+The consequence for the real persistence module is a **boot-recovery** rule: a
+lingering `*.tmp` at startup means the last save didn't finish → promote it
+(it's the newest complete copy). The spike now does unlink-then-rename and
+tolerates a missing target, so re-runs are idempotent; recorded in
+[ADR-007](../adr.md#adr-007-storage-split--fat-on-sd-for-working-copy-littlefs-on-flash-for-config).
+
 ## Summary
 
 Built the Spike 3 bench program (`sd_fat`): bring up the SD card over the EPD's
@@ -143,16 +178,22 @@ it is explicitly **not recommended** and not applied.
 
 ## Follow-ups
 
-- [ ] Re-run with a genuine ≤32 GB card → expect clean mount + round-trip.
-- [ ] Strip the debug logging once green: remove `CONFIG_LOG_MAXIMUM_LEVEL_DEBUG`
-      from `sdkconfig.defaults` and the `esp_log_level_set` block in `sd_fat.rs`.
-- [ ] Write up Spike 3 as verified in
-      [`../../firmware/README.md`](../../firmware/README.md) (wiring, LFN
-      requirement, card-compatibility note), matching the other spikes.
-- [ ] Record a **recommended-SD-card note** for v0.1 (genuine, ≤32 GB;
-      large/counterfeit SDXC may fail CMD59) — product doc / ADR-007.
-- [ ] Settle the **shared-bus arbitration** decision (EPD lock vs. SPI3 for SD).
-- [ ] Enable PSRAM, then build Spike 7 (gitoxide push) for the push leg.
+- [x] Re-run with a genuine ≤32 GB card → clean mount + round-trip (2026-07-11).
+- [x] Strip the debug logging once green: removed `CONFIG_LOG_MAXIMUM_LEVEL_DEBUG`
+      from `sdkconfig.defaults` and the `esp_log_level_set` block in `sd_fat.rs`
+      (2026-07-11).
+- [x] Write up Spike 3 as verified in
+      [`../../firmware/README.md`](../../firmware/README.md) (2026-07-11).
+- [x] Record a **recommended-SD-card note** for v0.1 (genuine, ≤32 GB;
+      large/counterfeit SDXC may fail CMD59) — in
+      [ADR-007](../adr.md#adr-007-storage-split--fat-on-sd-for-working-copy-littlefs-on-flash-for-config)
+      (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`.
+- [ ] **Still open:** implement the FatFS atomic-save (unlink-then-rename +
+      `*.tmp` boot-recovery) in the real `persistence` module.
 
 ## Artifacts (this session)