🔧 Fix all unused_mut warnings: 16 instances cleaned up

- Remove unnecessary 'mut' from variable declarations
- Clean up code in boxes/, interpreter/, mir/, backend/, and runtime/
- No functional changes, just cleaner code

src/interpreter/core.rs

@@ -98,7 +98,7 @@ impl NyashInterpreter {
             reg.register(udf);
         }
 
-        let mut this = Self {
+        let this = Self {
             shared,
             local_vars: HashMap::new(),
             outbox_vars: HashMap::new(),
@@ -168,7 +168,7 @@ impl NyashInterpreter {
             reg.register(udf);
         }
 
-        let mut this = Self {
+        let this = Self {
             shared,
             local_vars: HashMap::new(),
             outbox_vars: HashMap::new(),