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/main.rs

@@ -301,7 +301,7 @@ async fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
 match crate::transport::middle_proxy::fetch_proxy_secret(proxy_secret_path).await {
     Ok(proxy_secret) => {
         info!(
-            secret_len = proxy_secret.len() as usize,  // ← ЯВНЫЙ ТИП usize
+            secret_len = proxy_secret.len(),
             key_sig = format_args!(
                 "0x{:08x}",
                 if proxy_secret.len() >= 4 {
@@ -597,14 +597,12 @@ match crate::transport::middle_proxy::fetch_proxy_secret(proxy_secret_path).awai
 			} else {
 				info!("  IPv4 in use / IPv6 is fallback");
 			}
-		} else {
-			if v6_works && !v4_works {
-				info!("  IPv6 only / IPv4 unavailable)");
-			} else if v4_works && !v6_works {
-				info!("  IPv4 only / IPv6 unavailable)");
-			} else if !v6_works && !v4_works {
-				info!("  No DC connectivity");
-			}
+		} else if v6_works && !v4_works {
+			info!("  IPv6 only / IPv4 unavailable)");
+		} else if v4_works && !v6_works {
+			info!("  IPv4 only / IPv6 unavailable)");
+		} else if !v6_works && !v4_works {
+			info!("  No DC connectivity");
 		}
 
 		info!("  via {}", upstream_result.upstream_name);