HCMA Paper 4 Extension: Mondrian Conformal Correction for Conditional Coverage

Extension of HCMA Paper 4 (OSF GHVMZ): a Mondrian (group-conditional) conformal correction that restores conditional coverage in versioned retrieval. Paper 4's conformal abstention guarantee was marginal; stratifying by query difficulty exposed a conditional coverage gap (medium-margin group loss 0.226 vs α=0.10). This extension partitions queries by their retrieval margin (low/medium/high, tertiles of the calibration split) and calibrates a separate conformal threshold per group. Result: the medium-margin group loss drops from 0.226 to 0.043 (α=0.10), restoring conditional validity at the cost of coverage in the hardest group. The margin stratifies difficulty cleanly (accuracy 0.365 / 0.774 / 1.000 across groups), confirming that margin-based conditioning is sufficient — no version-count signal is required. Artifacts: mondrian_conformal.py + mondrian_conformal_results.json. Requires: HCMA core (Paper 1, OSF VQX95) and Paper 4 (OSF GHVMZ). License: CC-BY 4.0 (docs); Apache-2.0 (code, SPDX headers inline).