This PR adds support for UV (https://github.com/astral-sh/uv), a fast Python package installer and resolver, to the python-environment-tools library.

What this PR does

• Discovers UV-managed global Python installations: Detects Python installations managed by UV in its data directory
• Cross-platform support: Works on Windows, macOS, and Linux with platform-specific default locations
• Respects UV configuration: Honors the UV_DATA_DIR environment variable for custom UV data locations
• Integrates with existing virtual environment detection: UV-created virtual environments are standard Python venvs that are automatically detected by the existing venv locator

Implementation details

Core changes:

• Added PythonEnvironmentKind::Uv to represent UV-managed Python environments
• Added EnvManagerType::Uv to represent UV as an environment manager
• Added LocatorKind::Uv for the UV locator type
• Updated README.md to document UV as a supported environment type

New pet-uv crate:

• Implements the Locator trait for UV Python discovery
• Discovers Python installations in UV's data directory:
  • Windows: %APPDATA%\uv\python\
  • Unix-like: ~/.local/share/uv/python/
  • Custom: Configurable via UV_DATA_DIR environment variable
• Identifies UV Python installations by checking if executables/prefixes are under UV's python directory
• Finds UV executable in PATH for proper manager attribution

Testing:

• Comprehensive unit tests for UV data directory discovery
• Integration tests for end-to-end UV locator functionality
• All existing tests continue to pass
• Manual verification with mock UV environments

Usage example

# UV locator appears in breakdown
$ pet find
Breakdown by each locator:
--------------------------
Conda                : 19.507ms
...
Uv                   : 127.519µs
...

Environments (4):
------------------
Conda                : 1
LinuxGlobal          : 2
Uv                   : 1

# Can filter specifically for UV environments
$ pet find --kind uv

# UV appears as a supported environment kind
$ pet find --help
... --kind <KIND> ... [possible values: ..., uv, ...]

This addresses both requirements from the issue:

• ✅ Discovery of all global Python installs (done via UV)
• ✅ Discovery of all Python virtual environments (created via UV, detected as standard venvs)

Fixes #227.

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