QA re-smoke after #718 (fixup 2/3): canonical 'фывапролд 999' → ЕКБ
city-centroid still returned medium. Root cause: the coarse-geo gate guard
was anchor_tier is None, but a coarse geocode landing on the dense city
centroid lets Tier C (micro-radius ≤500 m) collect ≥5 comps → anchor_tier='C'
→ guard blocked the downgrade. Tier C is NOT same-building (#691); it is just
random flats within 500 m of a meaningless centroid — exactly the false
confidence this issue targets. Digit-free garbage ('ёъхзщшг') proves
_geocode_is_coarse already returned True; only the anchor guard blocked it.
Protect ONLY Tier A (real same-building comps); coarse geo + Tier C now
downgrades to low + flag. Add prod-diagnostic log of geo fields (QA request).
Refs #693
