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/externals.rs

@@ -155,7 +155,7 @@ impl MirInterpreter {
                 // Note: This branch is used for ExternCall form; provider toggles must be ON.
                 if std::env::var("NYASH_LLVM_USE_CAPI").ok().as_deref() != Some("1") ||
                    std::env::var("HAKO_V1_EXTERN_PROVIDER_C_ABI").ok().as_deref() != Some("1") {
-                    return Err(VMError::InvalidInstruction("env.codegen.link_object: C-API route disabled".into()));
+                    return Err(self.err_invalid("env.codegen.link_object: C-API route disabled"));
                 }
                 // Extract array payload
                 let (obj_path, exe_out) = if let Some(a2) = args.get(2) {
@@ -177,13 +177,13 @@ impl MirInterpreter {
                         _ => (v.to_string(), None),
                     }
                 } else {
-                    return Err(VMError::InvalidInstruction("extern_invoke env.codegen.link_object expects args array".into()));
+                    return Err(self.err_invalid("extern_invoke env.codegen.link_object expects args array"));
                 };
                 let extra = std::env::var("HAKO_AOT_LDFLAGS").ok();
                 let obj = std::path::PathBuf::from(obj_path);
                 let exe = exe_out.map(std::path::PathBuf::from).unwrap_or_else(|| std::env::temp_dir().join("hako_link_out.exe"));
                 crate::host_providers::llvm_codegen::link_object_capi(&obj, &exe, extra.as_deref())
-                    .map_err(|e| VMError::InvalidInstruction(format!("env.codegen.link_object: {}", e)))?;
+                    .map_err(|e| self.err_with_context("env.codegen.link_object", &e.to_string()))?;
                 self.write_result(dst, VMValue::String(exe.to_string_lossy().into_owned()));
                 Ok(())
             }
@@ -195,9 +195,9 @@ impl MirInterpreter {
                     }
                     return Ok(());
                 }
-                return Err(VMError::InvalidInstruction("hostbridge.extern_invoke unsupported [externals]".into()));
+                return Err(self.err_invalid("hostbridge.extern_invoke unsupported [externals]"));
             }
-            _ => Err(VMError::InvalidInstruction(format!(
+            _ => Err(self.err_invalid(format!(
                 "ExternCall {}.{} not supported",
                 iface, method
             ))),