proxy-entanglement.md says "six audits" and undercounts Healthcare Readmission's proxy cluster by one variable

[yakew7]

Where: explainers/proxy-entanglement.md, two separate staleness bugs:

1. Line ~27: calls Healthcare Readmission "the most structurally complex of all six audits." This repo has seven audit directories (COMPAS, Benefits Denial, German Credit Lending, Healthcare Readmission, Insurance Denial, Tenant Screening, AI Fair Recruitment) - the main README says "seven audits" in three places. Same drift pattern already fixed elsewhere this cycle for README.md still says "six" audits/projects in two places - it's seven since Tenant Screening shipped #353/README.md's Open Dataset Profiler intro still says "six audits" - it's seven since Tenant Screening shipped #592/profiler.html still says "six bias audits" - it's seven #624/README.md's "all six projects" line is missing Tenant Screening entirely #627.

2. Line ~45: states "Three proxy variables - payer_code, discharge_disposition_id, and number_inpatient - form an entangled cluster," omitting medical_specialty. This contradicts the rest of the same file: the "Fix" code block (~lines 77-99), the detection-code example output (~lines 185-199), and the "Related Projects" section (~line 235) all treat it as a four-variable cluster. Healthcare Readmission/audit.yaml's proxy_features list confirms four: payer_code, discharge_disposition_id, medical_specialty, number_inpatient.

Fix: "six" -> "seven" (item 1); "Three proxy variables" -> "Four proxy variables" and add medical_specialty to the named list (item 2), matching the rest of the file's own internally-consistent four-variable framing.