When v_prior == 0 and v_rec > 0, the old code unconditionally assigned
trend_ratio = 2.0, producing an artificial 2x jump even for districts
with negligible recent velocity.
New formula:
ratio = 1.0 + min(1.0, v_rec / _REF_VELOCITY) * (_TREND_CAP_VPRIOR_ZERO - 1.0)
Where:
_REF_VELOCITY = 10.0 (monthly flats/corpus — EKB "well-performing" benchmark)
_TREND_CAP_VPRIOR_ZERO = 1.5 (max ratio for the v_prior==0 case)
Tiny v_rec (e.g. 1 flat/month) → ratio ≈ 1.05 (near neutral)
Large v_rec (≥ 10 flat/month) → ratio → 1.5 (capped, below old hard 2.0)
v_prior > 0 branch is unchanged.
Also fixes pre-existing ruff violations in the same file (E401 multi-import,
E701 inline colon, E722 bare except, F401 unused import) so ruff check passes clean.
