#1511: replace equal-weight per-row median with volume-weighted median
(sorted by price, cumulative sold_volume_m2 weight, stop at 50th percentile).
Each corpus×month row now counts proportionally to its deal volume instead of
contributing equal weight regardless of how many flats were sold.
#1512: pin latest_stock to the single most-recent month in the window (last3[0])
instead of per-corpus ROW_NUMBER latest. Stale stock from inactive corpuses no
longer inflates the MTS numerator; stock and sold_volume_m2 denominator now
refer to the same consistent period.
Also clean pre-existing ruff E401/E701/E702/E722 violations (no logic change).
