Standalone bench program (src/bin/sd_fat.rs, `just flash-sd`) that mounts
FAT over the EPD's shared SPI2 bus and proves the persistence module's
atomic save: write .tmp, fsync, rename, read back and compare.
Runs SD-only: the EPD's SpiBusDriver holds an exclusive bus lock for its
lifetime, so an arbitrated SD device can't be live alongside it yet. Keeps
CRC required and maps a card that rejects CMD59 to a clear "use a genuine
card" message rather than running the user's notes over an unchecked bus.
sdkconfig gains CONFIG_FATFS_LFN_HEAP (the atomic-save .tmp name isn't valid
8.3) and, temporarily, CONFIG_LOG_MAXIMUM_LEVEL_DEBUG to read the drivers'
per-command R1 bytes during bring-up.
Standalone `wifi_tls` binary: station assoc, SNTP, then a validated
HTTPS GET to api.github.com against the esp-idf cert bundle. Gates
Spike 7 (gitoxide push). Creds come from firmware/.env via build.rs.
