Phase 5: Complete destination pattern unification (28 sites, 53 lines)

Unified remaining destination write patterns with new helpers:
- write_string() for VMValue::String writes
- write_from_box() for VMValue::from_nyash_box() patterns

Files updated:
- boxes.rs: 8 sites unified (-14 lines)
- boxes_plugin.rs: 9 sites unified (-17 lines)
- boxes_object_fields.rs: 7 sites unified (-14 lines)
- boxes_instance.rs: 2 sites unified (-4 lines)
- calls.rs: 2 sites unified (-4 lines)
- destination_helpers.rs: +28 lines (new helpers)

Impact: 28 sites unified, net -25 lines, improved maintainability
Tests: Phase 21.0 PASS (2/2, 100%)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/backend/mir_interpreter/utils/destination_helpers.rs

@@ -45,4 +45,32 @@ impl MirInterpreter {
             self.regs.insert(d, value);
         }
     }
+
+    /// String値をdestinationに書き込む
+    ///
+    /// # Arguments
+    /// * `dst` - 書き込み先のValueId (Noneの場合は何もしない)
+    /// * `value` - 書き込むString
+    #[inline]
+    pub(crate) fn write_string(&mut self, dst: Option<ValueId>, value: String) {
+        if let Some(d) = dst {
+            self.regs.insert(d, VMValue::String(value));
+        }
+    }
+
+    /// Box<dyn NyashBox>をVMValueに変換してdestinationに書き込む
+    ///
+    /// # Arguments
+    /// * `dst` - 書き込み先のValueId (Noneの場合は何もしない)
+    /// * `nyash_box` - 書き込むBox (NullBox→Void, IntegerBox→Integer等に自動変換)
+    #[inline]
+    pub(crate) fn write_from_box(
+        &mut self,
+        dst: Option<ValueId>,
+        nyash_box: Box<dyn crate::box_trait::NyashBox>,
+    ) {
+        if let Some(d) = dst {
+            self.regs.insert(d, VMValue::from_nyash_box(nyash_box));
+        }
+    }
 }