The read-only-attribute theory (and the predicted AM_RDO unlink shim) was
wrong. A diagnostic recursive walk showed every entry writable (ro=false)
and succeeded via path-based deletion, so the real blocker was
std::fs::remove_dir_all's openat/unlinkat/fdopendir usage against esp-idf's
path-based FATFS VFS. Update the increment-A/B checklist accordingly.
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