fix: audit improvements (SAFETY comment, warn on invalid params, lint config)

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.

Cargo.toml

@@ -29,7 +29,7 @@ embassy-sync = "0.8"
 embassy-time = "0.5"
 embassy-usb = { version = "0.6", features = ["defmt"] }
 embassy-usb-driver = "0.2"
-ucobs = "0.2"
+ucobs = { path = "../ucobs", features = ["legacy-msrv"] }
 heapless = "0.9"
 lora-phy = "3.0"
 static_cell = "2.1"
@@ -79,6 +79,9 @@ rak_wisblock_4631 = [
     "embassy-executor/executor-thread",
 ]
 
+[lints.clippy]
+undocumented_unsafe_blocks = "deny"
+
 [profile.release]
 codegen-units = 1
 lto = "fat"