Read-only SPI flash commands over the BSCAN proxy (via a small adapter
to spi_flash's xfer callback). flash_detect identifies the chip from
its JEDEC ID; flash_read hex-dumps a region.
Validated on the KCU105: flash_detect -> Micron MT25QU256, flash_read
shows the stored bitstream (sync word 0xAA995566 at 0x50). Erase/program
commands deferred to Phase 4.
François
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