Normalize real roots returned by CurveFunction

[SybelBlue]

Closes #31

Summary

This PR aligns CurveFunction.get_x_for_yval with its declared types:

• normalize accepted roots from CurveFunction.get_x_for_yval() to built-in float values
• use the configured imaginary-component tolerance when filtering complex results
• add regression coverage for returning the expected real value while omitting complex roots

Testing

Added a regression test and passed:

• uv run pytest -q
• uv run ruff check .
• uv run pyright

[nritschel]

I think there's a bug in your current change (see comment). Fixing it should be a matter of just replacing the comparison of xvals and tolerance to tvals. The rest looks good.

[nritschel]

Looks good now after the fix!