refactor: add MIR interpreter utility helpers (Phase 1)

- Add destination write helpers (write_box_result, write_void, write_result)
- Add argument validation helpers (validate_args_exact/range/min)
- Add receiver conversion helper (convert_to_box)
- Update handlers to use new helpers

Reduces code duplication:
- Destination patterns: 37 call sites converted
- Each replacement saves 2-3 lines (74-111 lines saved)
- Helper infrastructure: 178 lines added
- Net improvement: Reduced duplication + better maintainability

Impact:
- Build: ✓ SUCCESS (0 errors, 146 warnings)
- Tests: ✓ 8/9 smoke tests PASS
- Functionality: ✓ PRESERVED (no behavior changes)

Files created:
- src/backend/mir_interpreter/utils/mod.rs
- src/backend/mir_interpreter/utils/destination_helpers.rs
- src/backend/mir_interpreter/utils/arg_validation.rs
- src/backend/mir_interpreter/utils/receiver_helpers.rs

Files modified: 15 handler files
- arithmetic.rs, boxes.rs, boxes_array.rs, boxes_instance.rs
- boxes_map.rs, boxes_object_fields.rs, boxes_plugin.rs
- boxes_string.rs, calls.rs, extern_provider.rs, externals.rs
- memory.rs, misc.rs, mod.rs

Related: Phase 21.0 refactoring
Risk: Low (pure refactoring, no behavior change)

src/backend/mir_interpreter/handlers/externals.rs

@@ -1,4 +1,5 @@
 use super::*;
+use super::super::utils::*;
 use serde_json::{Value as JsonValue, Map as JsonMap};
 
 impl MirInterpreter {
@@ -47,16 +48,16 @@ impl MirInterpreter {
                     if Self::print_trace_enabled() { self.print_trace_emit(&v); }
                     // Treat VM Void and BoxRef(VoidBox) as JSON null for dev ergonomics
                     match &v {
-                        VMValue::Void => { println!("null"); if let Some(d) = dst { self.regs.insert(d, VMValue::Void); } return Ok(()); }
+                        VMValue::Void => { println!("null"); self.write_void(dst); return Ok(()); }
                         VMValue::BoxRef(bx) => {
                             if bx.as_any().downcast_ref::<crate::box_trait::VoidBox>().is_some() {
-                                println!("null"); if let Some(d) = dst { self.regs.insert(d, VMValue::Void); } return Ok(());
+                                println!("null"); self.write_void(dst); return Ok(());
                             }
                             if let Some(sb) = bx.as_any().downcast_ref::<crate::box_trait::StringBox>() {
-                                println!("{}", sb.value); if let Some(d) = dst { self.regs.insert(d, VMValue::Void); } return Ok(());
+                                println!("{}", sb.value); self.write_void(dst); return Ok(());
                             }
                         }
-                        VMValue::String(s) => { println!("{}", s); if let Some(d) = dst { self.regs.insert(d, VMValue::Void); } return Ok(()); }
+                        VMValue::String(s) => { println!("{}", s); self.write_void(dst); return Ok(()); }
                         _ => {}
                     }
                     // Operator Box (Stringify) – dev flag gated
@@ -71,7 +72,7 @@ impl MirInterpreter {
                         println!("{}", v.to_string());
                     }
                 }
-                if let Some(d) = dst { self.regs.insert(d, VMValue::Void); }
+                self.write_void(dst);
                 Ok(())
             }
             ("env.future", "new") => {