Phase 1: core/calibration.py - ECE/MCE + reliability + temperature/isotonic

[Rajveer-code]

ECE (uniform|quantile), MCE, ece_by_group, reliability_diagram; plus subgroup-stratified wrappers around standard recalibration: temperature scaling (Guo et al. 2017) and isotonic (Zadrozny & Elkan 2002). OvR for multiclass.