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_array.rs

@@ -32,7 +32,7 @@ pub(super) fn try_handle_array_box(
                     return Ok(true);
                 }
                 "pop" => {
-                    if !args.is_empty() { return Err(VMError::InvalidInstruction("pop expects 0 args".into())); }
+                    if !args.is_empty() { return Err(this.err_invalid("pop expects 0 args")); }
                     let ret = ab.pop();
                     this.write_result(dst, VMValue::from_nyash_box(ret));
                     return Ok(true);