SD driver: a dummy byte before the ready test; say why a write failed (#21)

The card "refused" a write about once in 2,000 multi-block writes: three
1.7 MB uploads in ten. Measured with a driver that records where it gives
up: every time, all blocks were accepted, and the status check after Stop
Tran came back as 0xFF or 0x1F. The driver tests for ready with the first
byte after selecting the card, which reads 0xFF before the card has
signalled busy, so CMD13 went out mid-programming. A dummy byte first, as
in ChaN's reference driver, and one after Stop Tran.

30 uploads in a row since, each read back by SHA-256, ten with the radio
listening: no fault. 10 MHz made no difference; the card stays at 20 MHz.

`info` shows the driver's write faults; `put` prints the step and the
card's answer when one happens. ADR 0007.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

README.md

@@ -85,7 +85,7 @@ The LoRa Scanner (docs/milestones/M3.md) listens with the Cap's radio and **neve
 | `irc say <buffer> <text>` | Types into a Buffer, commands included (`irc say 0 /join #test`) |
 | `irc dump` | Prints IRC status, memory, and the last lines of each Buffer |
 | `wifi status` | Prints Wi-Fi state, network, signal, clock and free heap |
-| `info` | Firmware, uptime, last start reason, memory, Wi-Fi, and both app slots with their versions and OTA states |
+| `info` | Firmware, uptime, last start reason, memory, Wi-Fi, the SD card and its write faults since boot, and both app slots with their versions and OTA states |
 | `tasks` | FreeRTOS tasks: state, priority, lowest free stack, CPU share |
 | `reboot` / `boot other` | Restart, or restart into the other app slot (a manual Rollback) |
 | `log level <0-5>` | ESP-IDF log level |