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/boxes_plugin.rs

@@ -1,4 +1,5 @@
 use super::*;
+use super::super::utils::*;
 use crate::box_trait::NyashBox;
 
 pub(super) fn invoke_plugin_box(
@@ -120,7 +121,7 @@ pub(super) fn invoke_plugin_box(
                             let j = (i + 1).min(bytes.len());
                             String::from_utf8(bytes[i..j].to_vec()).unwrap_or_default()
                         };
-                        if let Some(d) = dst { this.regs.insert(d, VMValue::String(s)); }
+                        this.write_result(dst, VMValue::String(s));
                         return Ok(());
                     }
                 }
@@ -148,14 +149,14 @@ pub(super) fn invoke_plugin_box(
                         Some(crate::value::NyashValue::String(ref s)) => s == "EOF",
                         _ => false,
                     };
-                    if let Some(d) = dst { this.regs.insert(d, VMValue::Bool(is)); }
+                    this.write_result(dst, VMValue::Bool(is));
                     return Ok(());
                 }
                 if inst.class_name == "JsonScanner" {
                     let pos = match inst.get_field_ng("position") { Some(crate::value::NyashValue::Integer(i)) => i, _ => 0 };
                     let len = match inst.get_field_ng("length")   { Some(crate::value::NyashValue::Integer(i)) => i, _ => 0 };
                     let is = pos >= len;
-                    if let Some(d) = dst { this.regs.insert(d, VMValue::Bool(is)); }
+                    this.write_result(dst, VMValue::Bool(is));
                     return Ok(());
                 }
             }
@@ -189,7 +190,7 @@ pub(super) fn invoke_plugin_box(
     if recv_box.type_name() == "VoidBox" {
         match method {
                 "object_get" | "array_get" | "toString" => {
-                    if let Some(d) = dst { this.regs.insert(d, VMValue::Void); }
+                    this.write_void(dst);
                     return Ok(());
                 }
                 "stringify" => {
@@ -197,12 +198,12 @@ pub(super) fn invoke_plugin_box(
                     return Ok(());
                 }
                 "array_size" | "length" | "size" => {
-                    if let Some(d) = dst { this.regs.insert(d, VMValue::Integer(0)); }
+                    this.write_result(dst, VMValue::Integer(0));
                     return Ok(());
                 }
                 "object_set" | "array_push" | "set" => {
                     // No-op setters on null receiver
-                    if let Some(d) = dst { this.regs.insert(d, VMValue::Void); }
+                    this.write_void(dst);
                     return Ok(());
                 }
                 _ => {}