Slice 3a. The inverse of Epc.from_sap_score: the minimum SAP rating in a
band (C → 69, B → 81, …), used as the Optimiser's repair target for an
INCREASING_EPC goal (goal_value "C" → target SAP 69). Keeps the
band-target derivation in the domain rather than re-coupling to
backend.app.utils.epc_to_sap_lower_bound. 8 tests incl. round-trip
through from_sap_score; pyright strict clean.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
