Fix ruff pyproject.toml deprecation and lint warnings

[Copilot]

pre-commit's ruff hook warned that top-level linter settings (select, ignore, isort) are deprecated in favor of the lint section, and flagged PLR0917 (too many positional arguments) on _get_stellar_spectrum.

Config migration

• Moved select/ignore from [tool.ruff] to [tool.ruff.lint]
• Moved isort settings from [tool.ruff.isort] to [tool.ruff.lint.isort]

PLR0917 suppression

• Rather than adding a repo-wide ignore for PLR0917, suppressed it locally with # noqa: PLR0917 on the _get_stellar_spectrum definition line, since this rule shouldn't be silenced globally for a single method's signature:

def _get_stellar_spectrum(  # noqa: PLR0917
    self,
    mact,
    logt,
    lbol,
    logg,
    phase,
    comp,
    mdot=0,
    weight=1,
    zmet=None,
    peraa=True,
):
    ...