fix: Resolve Rust 1.93.0 Clippy lints in zipherx-crypto

- Replace cloned_ref_to_slice_refs with std::slice::from_ref
- Remove drop_non_drop calls on non-Drop types
- Remove unnecessary u32-to-u32 casts
- Use is_multiple_of() instead of manual modulo checks
- Extract ParsedOutput type alias for complex tuple

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: VictorLux

crates/zipherx-crypto/src/boost_scan.rs

@@ -8,6 +8,9 @@
 
 use std::collections::{HashMap, HashSet};
 
+/// Parsed output record: (array_idx, height, index_field, cmu, epk, ciphertext, received_txid)
+type ParsedOutput = (usize, u32, u32, [u8; 32], [u8; 32], [u8; 580], [u8; 32]);
+
 use ff::PrimeField;
 use rayon::prelude::*;
 use serde::{Deserialize, Serialize};
@@ -198,7 +201,7 @@ pub fn scan_boost_outputs(
 
     // Parse output at index i directly from raw bytes — avoids ~700MB intermediate allocation.
     // Returns: (array_idx, height, index_field, cmu, epk, ciphertext, received_txid)
-    let parse_output = |i: usize| -> (usize, u32, u32, [u8; 32], [u8; 32], [u8; 580], [u8; 32]) {
+    let parse_output = |i: usize| -> ParsedOutput {
         let offset = i * BOOST_OUTPUT_SIZE;
         let height = u32::from_le_bytes([
             outputs_data[offset],

crates/zipherx-crypto/src/keys.rs

@@ -36,11 +36,9 @@ pub fn derive_spending_key(seed: &[u8], account: u32) -> Result<Zeroizing<Vec<u8
         .write(&mut buf)
         .map_err(|e| CryptoError::InvalidData(format!("SK serialize: {e}")))?;
 
-    // Note: explicit drop() ends the scope of these temporaries but does NOT
-    // zero memory. The actual zeroization is provided by the Zeroizing<Vec<u8>>
-    // return type, which zeros on drop via the zeroize crate.
-    drop(account_key);
-    drop(master);
+    // Note: `account_key` and `master` are dropped here at scope end.
+    // Their types do NOT implement Drop (no zeroization), but the actual
+    // secret bytes are returned via Zeroizing<Vec<u8>>, which zeros on drop.
 
     if buf.len() != SPENDING_KEY_LENGTH {
         return Err(CryptoError::InvalidData(format!(

crates/zipherx-crypto/src/prover.rs

@@ -121,19 +121,19 @@ pub(crate) fn get_verifying_keys(
 /// Get the total number of proofs to generate for the current TX.
 pub fn proof_total() -> u32 {
     zcash_primitives::transaction::components::sapling::builder::GROTH16_PROOFS_TOTAL
-        .load(std::sync::atomic::Ordering::Relaxed) as u32
+        .load(std::sync::atomic::Ordering::Relaxed)
 }
 
 /// Get the number of proofs completed so far.
 pub fn proof_completed() -> u32 {
     zcash_primitives::transaction::components::sapling::builder::GROTH16_PROOFS_COMPLETED
-        .load(std::sync::atomic::Ordering::Relaxed) as u32
+        .load(std::sync::atomic::Ordering::Relaxed)
 }
 
 /// Get the number of proof threads in use.
 pub fn proof_threads() -> u32 {
     zcash_primitives::transaction::components::sapling::builder::GROTH16_PROOF_THREADS
-        .load(std::sync::atomic::Ordering::Relaxed) as u32
+        .load(std::sync::atomic::Ordering::Relaxed)
 }
 
 /// Cancel ongoing proof generation.

crates/zipherx-crypto/src/transaction.rs

@@ -103,7 +103,7 @@ pub fn build_transaction(
         to_address,
         amount,
         memo,
-        &[spend.clone()],
+        std::slice::from_ref(spend),
         chain_height,
     )
 }
@@ -343,14 +343,13 @@ pub fn build_transaction_multi(
         )));
     }
 
-    // RCR-6: SECURITY — `extsk` is dropped here as defense-in-depth, but Rust's
-    // default `Drop` does NOT zero memory. The `&[u8]` slice received here cannot
-    // be zeroed by this function (immutable borrow).
+    // RCR-6: SECURITY — `extsk` goes out of scope here, but Rust's default Drop
+    // does NOT zero memory. The `&[u8]` slice received here cannot be zeroed by
+    // this function (immutable borrow).
     //
     // IMPORTANT: Callers MUST wrap `sk_bytes` in `zeroize::Zeroizing<Vec<u8>>` to
     // ensure spending key material is securely zeroed on drop. Without `Zeroizing`,
     // key material may persist in freed memory.
-    drop(extsk);
 
     Ok(TransactionResult {
         tx_bytes,

crates/zipherx-crypto/src/tree.rs

@@ -160,7 +160,7 @@ pub fn append(cmu: &[u8; 32]) -> Result<u64, CryptoError> {
 ///
 /// Returns the new tree size.
 pub fn append_batch(cmus: &[u8]) -> Result<u64, CryptoError> {
-    if cmus.len() % 32 != 0 {
+    if !cmus.len().is_multiple_of(32) {
         return Err(CryptoError::InvalidData(format!(
             "CMU data length {} not a multiple of 32",
             cmus.len()

crates/zipherx-crypto/src/witness.rs

@@ -84,7 +84,7 @@ pub fn update_witness(witness_data: &[u8], cmu: &[u8; 32]) -> Result<Vec<u8>, Cr
 ///
 /// Returns the updated witness bytes.
 pub fn update_witness_batch(witness_data: &[u8], cmus: &[u8]) -> Result<Vec<u8>, CryptoError> {
-    if cmus.len() % 32 != 0 {
+    if !cmus.len().is_multiple_of(32) {
         return Err(CryptoError::InvalidData(format!(
             "CMU data length {} not a multiple of 32",
             cmus.len()