Anchor over-priced cheaper/smaller units in flagship buildings (Хохрякова 48
3к/153: real 61M, est 100M). Two conservative levers: (1) tighten premium p70
uplift gate to area>=p66 AND rooms>=building-median (a 3-room in a 4-room-flagship
building no longer inherits the flagship ppm2); (2) floor-similarity Gaussian on
relative position floor/total_floors (estimate_sb_floor_sigma=0.25, 0 disables).
Offline-validated on 55 golden cases: zero coverage flips, elite medAPE 16.6->5.4,
3к/153 overshoot 64%->1.5%; flagships (Хохрякова 4к, Олимп пентхаус) stay covered.
ruff clean; 34 anchor tests + 259 broader green.
Refs #651#677
