fix: forward :python option from compile-time config to uv fetch/init

[AJReade]

Summary

• The :python option (e.g., "3.14t" for free-threaded builds) set via config :pythonx, :uv_init is read from Application.compile_env but never forwarded to Pythonx.Uv.fetch/3 or Pythonx.Uv.init/3
• This causes uv sync to download the regular Python build instead of the requested variant

Context

PR #45 added the :python option to Pythonx.uv_init/2 (the runtime function), which works correctly. However, the compile-time config path in application.ex was not updated to forward this option. Users following the recommended Mix project setup via config :pythonx, :uv_init silently get the wrong Python variant.

Change

One line in lib/pythonx/application.ex — read :python from the config and include it in the opts passed to fetch/3 and init/3:

# Before
opts = [uv_version: uv_version]

# After
python = uv_init_env[:python]
opts = [uv_version: uv_version, python: python]

Verification

Tested on macOS aarch64. Before the fix, config :pythonx, :uv_init, python: "3.14t" downloads cpython-3.14.3-macos-aarch64-none (regular). After the fix, it downloads cpython-3.14.3+freethreaded-macos-aarch64-none and confirms GIL disabled.