feat(alknet-secret): make DerivedKey zeroize-on-drop, non-Clone, with redacted serialization

Per ADR-038, DerivedKey.private_key now derives Zeroize with #[zeroize(drop)]
ensuring sensitive key material is zeroized before deallocation. DerivedKey
is now move-only (no Clone), and JSON/debug output redacts private_key as
"[REDACTED]". Deserialization still works for postcard/irpc wire format.

Also fixes clippy needless_borrows_for_generic_args in encryption.rs and
applies cargo fmt to existing code.

Cargo.lock

@@ -138,6 +138,7 @@ dependencies = [
  "hmac",
  "irpc",
  "irpc-derive",
+ "postcard",
  "rand 0.8.6",
  "secp256k1",
  "serde",
@@ -389,6 +390,15 @@ dependencies = [
  "webpki-roots 0.26.11",
 ]
 
+[[package]]
+name = "atomic-polyfill"
+version = "1.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8cf2bce30dfe09ef0bfaef228b9d414faaf7e563035494d7fe092dba54b300f4"
+dependencies = [
+ "critical-section",
+]
+
 [[package]]
 name = "atomic-waker"
 version = "1.1.2"
@@ -1746,6 +1756,15 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "hash32"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b0c35f58762feb77d74ebe43bdbc3210f09be9fe6742234d573bacc26ed92b67"
+dependencies = [
+ "byteorder",
+]
+
 [[package]]
 name = "hashbrown"
 version = "0.14.5"
@@ -1769,6 +1788,20 @@ version = "0.17.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ed5909b6e89a2db4456e54cd5f673791d7eca6732202bbf2a9cc504fe2f9b84a"
 
+[[package]]
+name = "heapless"
+version = "0.7.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cdc6457c0eb62c71aac4bc17216026d8410337c4126773b9c5daba343f17964f"
+dependencies = [
+ "atomic-polyfill",
+ "hash32",
+ "rustc_version",
+ "serde",
+ "spin 0.9.8",
+ "stable_deref_trait",
+]
+
 [[package]]
 name = "heck"
 version = "0.5.0"
@@ -3548,6 +3581,7 @@ dependencies = [
  "cobs",
  "embedded-io 0.4.0",
  "embedded-io 0.6.1",
+ "heapless",
  "postcard-derive",
  "serde",
 ]