Add multi-seed angular RG error bars and zoomed plots

[charlesmartin14]

Summary

• make both MuonClip angular notebooks analyze all completed seeds rather than one seed
• auto-discover seed directories with actual initial/best/final checkpoints, or accept an explicit ANGULAR_SEEDS list
• retain every seed-level result and compute mean ± 95% Student-t confidence intervals across training seeds
• never treat random-null realizations as independent seeds
• compare trained metrics with matched random-angular null medians across seeds
• add full-range and zoomed-y figures for alpha, tail length, power-law KS D, and package-selected xmin
• keep individual seed points visible on every aggregate plot
• keep alpha=2 visible in zoomed alpha plots
• preserve the existing powerlaw.Fit contract: all positive continuous values, package-selected xmin, no supplied xmax
• add notebook and aggregation regression tests

The zoomed y axis is display-only and does not alter any spectrum, tail fit, or null distribution.