refactor: complete error message unification (Phase 3.5)

Migrate remaining 45 error generation patterns to unified helpers:
- calls.rs: 13 sites → 0
- extern_provider.rs: 9 sites → 0
- externals.rs: 5 sites → 0
- boxes_plugin.rs: 5 sites → 0
- boxes.rs: 5 sites → 0
- boxes_string.rs: 4 sites → 0
- boxes_instance.rs: 2 sites → 0
- mod.rs + boxes_array.rs: 2 sites → 0

Error patterns now 100% unified:
- All 80 InvalidInstruction sites use helpers (100%)
- Consistent error formatting across entire codebase
- Single source of truth for error messages

Code reduction:
- Phase 3.5: 50-70 lines saved
- Cumulative (Phase 1+2+3+3.5): 200-267 lines removed (6-8% handlers)
- Total patterns unified: 192 (destination 60 + args 52 + errors 80)

Benefits:
- 100% error message consistency achieved
- Easy to modify error formats globally
- Foundation for i18n support ready
- Improved maintainability and testability

Test results: ✓ Build successful, Phase 21.0 smoke tests passing
Related: Phase 21.0 refactoring milestone complete
Risk: Low (error messages only, behavior preserved)

src/backend/mir_interpreter/handlers/boxes.rs

@@ -11,7 +11,7 @@ impl MirInterpreter {
     ) -> Result<(), VMError> {
         // Provider Lock guard (受け口・既定は挙動不変)
         if let Err(e) = crate::runtime::provider_lock::guard_before_new_box(box_type) {
-            return Err(VMError::InvalidInstruction(e));
+            return Err(self.err_invalid(e));
         }
         let mut converted: Vec<Box<dyn NyashBox>> = Vec::with_capacity(args.len());
         for vid in args {
@@ -22,9 +22,7 @@ impl MirInterpreter {
             .lock()
             .unwrap()
             .create_box(box_type, &converted)
-            .map_err(|e| {
-                VMError::InvalidInstruction(format!("NewBox {} failed: {}", box_type, e))
-            })?;
+            .map_err(|e| self.err_with_context(&format!("NewBox {}", box_type), &e.to_string()))?;
         // Store created instance first so 'me' can be passed to birth
         let created_vm = VMValue::from_nyash_box(created);
         self.regs.insert(dst, created_vm.clone());
@@ -74,10 +72,10 @@ impl MirInterpreter {
                     }
                 }
                 Err(e) => {
-                    return Err(VMError::InvalidInstruction(format!(
-                        "PluginInvoke {}.{} failed: {:?}",
-                        p.box_type, method, e
-                    )))
+                    return Err(self.err_with_context(
+                        &format!("PluginInvoke {}.{}", p.box_type, method),
+                        &format!("{:?}", e)
+                    ))
                 }
             }
             Ok(())
@@ -88,11 +86,7 @@ impl MirInterpreter {
             }
             Ok(())
         } else {
-            Err(VMError::InvalidInstruction(format!(
-                "PluginInvoke unsupported on {} for method {}",
-                recv_box.type_name(),
-                method
-            )))
+            Err(self.err_method_not_found(&recv_box.type_name(), method))
         }
     }
 
@@ -188,7 +182,7 @@ impl MirInterpreter {
         }
         if user_instance_class.is_some() && !crate::config::env::vm_allow_user_instance_boxcall() {
             let cls = user_instance_class.unwrap();
-            return Err(VMError::InvalidInstruction(format!(
+            return Err(self.err_invalid(format!(
                 "User Instance BoxCall disallowed in prod: {}.{} (enable builder rewrite)",
                 cls, method
             )));