Add PEP 735 dependency group support to UvBuilder

[yzx9]

• Add group(String...) method to UvBuilder that maps to uv sync --group, enabling PEP 735 dependency groups selection
• Groups are tracked in appose.json only when non-empty, preserving backward compatibility with existing environments
• Validate that group() is only used with pyproject.toml scheme

closes apposed/appose#32

Example

Environment env = Appose.uv()
    .content(pyprojectContent)
    .scheme("pyproject.toml")
    .group("appose")
    .group("cowsay")  # support multiple groups
    .build();

Test plan

• testUvPyprojectWithGroup: builds env with --group cowsay and verifies cowsay is available
• testUvGroupRejectsWithoutPyproject: confirms IllegalArgumentException when using group() without pyproject.toml
• testUvStateNoGroupField: verifies appose.json has no groups field when none specified (backward compat)
• testUvStateGroupFieldPresent: verifies appose.json contains groups when specified

All 7 UvBuilderTest tests pass