🔧 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(),

src/interpreter/objects/fields.rs

@@ -102,7 +102,7 @@ impl NyashInterpreter {
         let mut all_methods = HashMap::new();
         for parent_name in &box_decl.extends {
             use crate::box_trait::is_builtin_box;
-            let mut is_builtin = is_builtin_box(parent_name);
+            let is_builtin = is_builtin_box(parent_name);
             #[cfg(all(feature = "gui", not(target_arch = "wasm32")))]
             {
                 if parent_name == "EguiBox" { is_builtin = true; }