SharedSPI.h: FreeRTOS mutex protecting the shared SPI bus (pins
33/34/35) used by LoRa (SX1262), SD card, and future NFC.
sx126x.cpp: All 6 SPI transaction blocks wrapped with
shared_spi_mutex acquire/release. LoRa uses portMAX_DELAY
(always gets access, just delayed by SD if needed).
USBSD.h: USB MSC SD card access uses shared_spi_mutex with
200ms timeout. Deferred init (3s after boot) for SPI bus
readiness. Metrics: sd_ready, sd_reads, sd_fails counters.
IMULogger.h, Gui.h: SD card writes wrapped in shared_spi_mutex.
Status: SD card correctly reports 29.7GB via USB MSC. Reads
work intermittently (~50% success). Root cause: main loop
takes ~700ms per iteration, causing mutex timeout for MSC
callbacks. Needs loop time investigation to achieve reliable
USB mass storage.
The CO5300 display uses a separate SPI3 bus — not affected.
