refactor(handlers): extract boxes_object_fields module (Phase 2-1)

Split handlers/boxes.rs (1008→648 lines, -360 lines, -35.7%)

Changes:
- Created boxes_object_fields.rs (359 lines)
  - Extracted try_handle_object_fields function
  - Handles getField/setField for InstanceBox
- Updated mod.rs to declare new module
- Updated boxes.rs to redirect to new module
- Build verified: 0 errors, 85 warnings

Progress:
- Phase 1: Legacy deletion (2,997 lines)
- Phase 2-1: boxes_object_fields split (360 lines)
- Total reduction so far: 3,357 lines

Next: Phase 2-2 (boxes_instance.rs), Phase 2-3 (boxes_plugin.rs)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/backend/mir_interpreter/handlers/mod.rs

@@ -5,6 +5,7 @@ mod boxes;
 mod boxes_array;
 mod boxes_string;
 mod boxes_map;
+mod boxes_object_fields;
 mod calls;
 mod externals;
 mod memory;