Completes the flash command set over the BSCAN proxy:
- flash_erase <dev> <addr> <len> : erase covering sectors (destructive)
- flash_write <dev> <addr> <file> : program from a file (no auto-erase)
- flash_verify <dev> <addr> <file>: compare flash against a file
- flash_read gains an optional [file] arg for a raw binary dump (backup)
Validated on the KCU105 with a save -> erase -> write-random -> verify
-> erase -> restore round-trip (region left untouched). Apparent write
throughput ~100 KB/s once the proxy is loaded.