fix: resolve clippy warnings

Reduce clippy warnings from54 to16 by fixing mechanical issues:

- collapsible_if: collapse nested if-let chains with let-chains
- clone_on_copy: remove unnecessary .clone() on Copy types
- manual_clamp: replace .max().min() with .clamp()
- unnecessary_cast: remove redundant type casts
- collapsible_else_if: flatten else-if chains
- contains_vs_iter_any: replace .iter().any() with .contains()
- unnecessary_closure: replace .or_else(|| x) with .or(x)
- useless_conversion: remove redundant .into() calls
- is_none_or: replace .map_or(true, ...) with .is_none_or(...)
- while_let_loop: convert loop with if-let-break to while-let

Remaining16 warnings are design-level issues (too_many_arguments,
await_holding_lock, type_complexity, new_ret_no_self) that require
architectural changes to fix.

src/protocol/constants.rs

@@ -160,7 +160,7 @@ pub const MAX_TLS_CHUNK_SIZE: usize = 16384 + 256;
 
 /// Secure Intermediate payload is expected to be 4-byte aligned.
 pub fn is_valid_secure_payload_len(data_len: usize) -> bool {
-    data_len % 4 == 0
+    data_len.is_multiple_of(4)
 }
 
 /// Compute Secure Intermediate payload length from wire length.
@@ -179,7 +179,7 @@ pub fn secure_padding_len(data_len: usize, rng: &SecureRandom) -> usize {
         is_valid_secure_payload_len(data_len),
         "Secure payload must be 4-byte aligned, got {data_len}"
     );
-    (rng.range(3) + 1) as usize
+    rng.range(3) + 1
 }
 
 // ============= Timeouts =============
@@ -231,7 +231,6 @@ pub static RESERVED_NONCE_CONTINUES: &[[u8; 4]] = &[
 // ============= RPC Constants (for Middle Proxy) =============
 
 /// RPC Proxy Request
-
 /// RPC Flags (from Erlang mtp_rpc.erl)
 pub const RPC_FLAG_NOT_ENCRYPTED: u32 = 0x2;
 pub const RPC_FLAG_HAS_AD_TAG: u32    = 0x8;