find_arc_lines uncertainty handling fix

[hpparvi]

This PR fixes two issues in how the find_arc_lines handles uncertainties:

1. the find_arc_lines function now allows uncertainties to be any of Astropy's uncertainty types (it allowed previously only StdDevUncertainty)

2. If the spectrum doesn't include uncertainties, the uncertainties are estimated using a robust approach based on the DER_SNR algorithm by Stoerh et al. (2008). (with a small difference that we're using Astropy's sigma-clipped median instead of plain median).

I've also added the noise estimation function as specreduce.utils.utils.measure_noise.

AI/LLM Disclaimer: The same as in #319. I've used Claude Code with Fable 5.1 to co-develop the PR, but I understand what the code does and how it works, and can explain it when needed.

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.56%. Comparing base (742264f) to head (fa7adef).

Files with missing lines	Patch %	Lines
specreduce/utils/utils.py	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #321      +/-   ##
==========================================
+ Coverage   92.52%   92.56%   +0.03%     
==========================================
  Files          18       18              
  Lines        2341     2367      +26     
==========================================
+ Hits         2166     2191      +25     
- Misses        175      176       +1     

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