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_plugin.rs

@@ -60,10 +60,10 @@ pub(super) fn invoke_plugin_box(
                 }
                 Ok(())
             }
-            Err(e) => Err(VMError::InvalidInstruction(format!(
-                "BoxCall {}.{} failed: {:?}",
-                p.box_type, method, e
-            ))),
+            Err(e) => Err(this.err_with_context(
+                &format!("BoxCall {}.{}", p.box_type, method),
+                &format!("{:?}", e)
+            )),
         }
     } else if let Some(string_box) = recv_box
         .as_any()
@@ -80,9 +80,7 @@ pub(super) fn invoke_plugin_box(
                     }
                     Ok(())
                 } else {
-                    Err(VMError::InvalidInstruction(
-                        "lastIndexOf requires 1 argument".into(),
-                    ))
+                    Err(this.err_invalid("lastIndexOf requires 1 argument"))
                 }
             }
             "indexOf" | "find" => {
@@ -94,15 +92,10 @@ pub(super) fn invoke_plugin_box(
                     }
                     Ok(())
                 } else {
-                    Err(VMError::InvalidInstruction(
-                        "indexOf/find requires 1 argument".into(),
-                    ))
+                    Err(this.err_invalid("indexOf/find requires 1 argument"))
                 }
             }
-            _ => Err(VMError::InvalidInstruction(format!(
-                "BoxCall method {} not supported on StringBox",
-                method
-            ))),
+            _ => Err(this.err_method_not_found("StringBox", method)),
         }
     } else {
         // Special-case: minimal runtime fallback for common InstanceBox methods when
@@ -209,10 +202,6 @@ pub(super) fn invoke_plugin_box(
                 _ => {}
             }
         }
-        Err(VMError::InvalidInstruction(format!(
-            "BoxCall unsupported on {}.{}",
-            recv_box.type_name(),
-            method
-        )))
+        Err(this.err_method_not_found(&recv_box.type_name(), method))
     }
 }