Built the property-753950 cert (worklist P4, PR #1503) in the live RdSAP-10
Online tool via Playwright: SAP 44 (E), vs our engine's 42 and the lodged
2014 cert's 45 — a normal RdSAP-2012->SAP-10.2 recalculation spread, not a
defect surfaced by the multiple_glazed_proportion fix.
Also surfaced (and documented as a caveat, not fixed here) a pre-existing
SAP-16.2 gap: the cert lodges party_wall_length_m=0 on both floors despite
being semi-detached, which Elmhurst's validation rejects and this PR's
engine change doesn't touch.
