Ports the single unique commit from the abandoned
feat/tradein-hedonic-ground-floor branch (fe69d6e7) — floor==1
discount term in the hedonic composite, completing the year+floor+area
roadmap item from #2002 (year+area already shipped). The 356 commits
since branch-off never touched this code path: the port is byte-
identical to the original, verified against the abandoned commit and
against a hermetic backtest replay (median_bias_pct deltas match
fe69d6e7 exactly).
Live-DB backtest (--resolve-house-id) not run — no prod DB access from
this worker; hermetic fixture-based regression gate + unit tests cover
the merge bar.
