Handle binning mismatches in cross-spectrum script

[ajhewitt]

Summary

• detect when preregistered bin definitions disagree with the bandpower count and fall back to CLI-style bins
• infer fallback binning parameters from the order spectra metadata when available and assume lmin=0 if none is provided

Testing

• pytest

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https://chatgpt.com/codex/tasks/task_e_68f70c55f70c832fbccb54e586e5db3f