Phase 6 WIP: Type conversion helpers (11 sites, 23 lines)

Added conversion_helpers.rs with unified type conversion:
- load_as_box() for reg_load().to_nyash_box()
- load_as_string() for reg_load().to_string()
- load_as_int/bool() for type-checked loads
- load_args_as_boxes/values() for bulk conversion

Files updated (partial):
- boxes.rs: 2 sites (-6 lines)
- calls.rs: 5 sites (-10 lines)
- boxes_plugin.rs: 1 site (-3 lines)
- externals.rs: 3 sites (-4 lines)

Next: boxes_array, boxes_map, boxes_object_fields, boxes_instance

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/mod.rs

@@ -4,9 +4,11 @@ pub mod destination_helpers;
 pub mod arg_validation;
 pub mod receiver_helpers;
 pub mod error_helpers;
+pub mod conversion_helpers;
 
 // Re-export for convenience
 pub use destination_helpers::*;
 pub use arg_validation::*;
 pub use receiver_helpers::*;
 pub use error_helpers::*;
+pub use conversion_helpers::*;