refactor(handlers): extract boxes_instance module (Phase 2-2)

Split handlers/boxes.rs (648→497 lines, -151 lines, -23.3%)

Changes:
- Created boxes_instance.rs (150 lines)
  - Extracted try_handle_instance_box function
  - Handles InstanceBox method dispatch
  - Supports Class.method/arity and ClassInstance.method/arity
- 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)
- Phase 2-2: boxes_instance split (151 lines)
- Total reduction so far: 3,508 lines

Next: Phase 2-3 (boxes_plugin.rs, ~176 lines)

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

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

src/backend/mir_interpreter/handlers/mod.rs

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