feat(train): ADR-145 ablation eval harness + privacy-leakage/latency metrics (#849)

- train/ablation.rs: FeatureSet matrix (CSI/CIR/CSI+CIR/+Doppler/+BFLD/+UWB);
  AblationMetrics (presence acc, loc err, FP/FN, latency p50/p95, privacy
  leakage, cross-room degradation) derived deterministically from VariantRun
- membership_inference_leakage(): MIA proxy = |AUC-0.5|*2 (0 indistinguishable,
  1 perfectly separable); latency_percentiles_ms (nearest-rank); confusion_rates
- AblationReport.to_markdown() (deterministic), csi_cir_beats_csi_only()
  acceptance check
- 5 tests; workspace 0 errors

Co-Authored-By: claude-flow <ruv@ruv.net>