Add reduced-chi2, KL, and whitened-signal density diagnostic plots

[hombit]

Summary

Adds per-band diagnostic plots that compare a survey's reported vs. model-corrected photometric uncertainties, in the same style as the existing whitened-signal plots.

• Reduced χ² — plot_chi2_distributions / make_chi2_distribution_plot: per-band lg(reduced χ²) distribution of full light curves, before and after correction, against a calibrated reference (the χ²_{N-1}/(N-1) mixture over the empirical light-curve-length distribution).
• KL statistic — plot_kl_distributions / make_kl_distribution_plot: per-band lg(KL) of the per-light-curve KLWhitenLc statistic ½(μ² + σ² − ln σ² − 1), against a Monte-Carlo calibrated reference.
• Density heatmaps — plot_whiten_density / plot_addmagerr_density: P(z | mag) and P(Δm | mag) per band (2D companions to the whitened-signal histograms).
• Drivers — scripts/plot_reduced_chi2.py, scripts/plot_kl.py: load a trained model, reconstruct its input columns, and render the figures.

Notes

• The reduced χ² is computed with the O(n) inverse-variance-weighted formula, which is identically Σ_j z_j²/(N−1); no whitening matrix is built.
• Uncertainties are corrected by the model's error-scaling factor only (no signal correction).
• Colors/layout follow plot_whiten_distributions (uncorrected = filled, corrected = line; colorblind-safe tableau-colorblind10).
• Verified end-to-end on DP1 forced and difference-image photometry (test split) on all six bands.

[codecov]

Codecov Report

❌ Patch coverage is 7.94521% with 336 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.22%. Comparing base (4a2e847) to head (008ec4d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uncle_val/pipelines/plotting.py	7.94%	336 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #103      +/-   ##
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- Coverage   63.18%   53.22%   -9.96%     
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  Files          32       32              
  Lines        1654     2018     +364     
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+ Hits         1045     1074      +29     
- Misses        609      944     +335     

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