chore: Upgrade build dependency to v4.0 for build_runner 2.10.0 compatibility

[SoutaTanaka]

Summary

Update the build package dependency from ^3.0.0 to ^4.0.0 to enable compatibility with the latest build_runner version.

Background

The build_runner 2.10.0 release introduced significant performance improvements through AOT (Ahead-of-Time) compiler optimizations. However, projects using index_generator were unable to upgrade to build_runner 2.10.0 due to dependency conflicts - index_generator was constrained to build version 3.x, while build_runner 2.10.0+ requires build version 4.x.

This update resolves the version conflict and allows users to benefit from the build_runner performance improvements.

Changes

• ✅ Updated build dependency from ^3.0.0 to ^4.0.0 in pubspec.yaml
• ✅ Updated recommended build_runner version in README from ^2.3.3 to ^2.10.0
• ✅ All existing tests pass with no code changes required

Motivation

• Unlock access to build_runner 2.10.0's AOT compiler performance benefits
• Resolve dependency conflicts preventing users from upgrading their build toolchain
• Maintain compatibility with the latest Dart ecosystem tooling

Backward Compatibility

• ✅ Fully backward compatible - no breaking changes to the public API
• ✅ No changes to generated code output
• ✅ Existing configurations continue to work without modification

Testing

• ✅ All existing unit tests pass
• ✅ Verified compatibility with build_runner 2.10.0
• ✅ No behavioral changes observed in code generation