Add SAFETY comment to nRF52840 FICR unsafe block explaining the
register addresses and why read_volatile is correct.
Replace silent fallback defaults in to_sf()/to_cr() with defmt::warn
so invalid values are logged instead of silently accepted.
Remove unnecessary #[allow(dead_code)] on DisplayCommand enum.
Add clippy::undocumented_unsafe_blocks = "deny" lint to Cargo.toml.