Use Hadris without the standard library

Disable default features, then select the platform, I/O mode, and capabilities that the target needs:

[dependencies]
hadris-fat = {
  version = "2.0.0",
  default-features = false,
  features = ["read", "sync"]
}

Add alloc for APIs backed by Vec, String, or owned trees. Add write only when mutation or image creation is required. std implies allocation but does not implicitly select sync or async.

All storage I/O flows through hadris-io, allowing callers to adapt firmware, kernel, memory, or device-specific readers rather than depending on std::io.