fix: fall back to initial kernel release tarball

[slegarraga]

Closes #25

Summary

Fixes kconfig kernel fetch X.Y.0 on a kernel's initial release.

kernel.org names an initial release linux-X.Y.tar.xz, with no trailing .0. The fetch command now tries the exact requested form first and falls back to the initial-release form when the first URL returns 404.

The fallback also normalizes the cached directory back to the requested version, so a later kconfig kernel fetch 6.8.0 sees the kernel as cached instead of downloading it again.

Tests

Added tests/test_kernel.py with:

• initial-release URL list includes the fallback;
• patch releases keep a single URL;
• the fetch flow falls back to the initial-release tarball and normalizes the cache directory.

Verification

uv run ruff check kconfig/cli/kernel.py tests/test_kernel.py
uv run mypy kconfig/cli/kernel.py
uv run pytest tests/test_kernel.py -q --no-cov

All pass.

Signed off with DCO.