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

src/services/storage_service.h

@@ -56,6 +56,7 @@ class StorageService : public Service {
     static constexpr uint32_t kWakeMs = 1000;
     static constexpr uint32_t kPollEvery = 15;  // wake-ups between usage checks
     static constexpr size_t kMaxPendingBytes = 16 * 1024;
+    static constexpr uint32_t kSdHz = 20000000;  // the card's SPI clock; 10 MHz changed nothing (ADR 0007)
 
     static void taskEntry(void* self);
     void loop();