PR #519 added cohort hard-filter but only to Tier W's inline SQL. Tier S
(address ILIKE) and Tier H (year ±15) share _COMMON_WHERE which was not
extended — for target_year=1978 (brezhnev), Tier H allowed year_built ∈
[1963, 1993] and a 1965 khrushchev or 1992 late_soviet match could win
before cohort filter ever evaluated.
Fixes (reviewer feedback on PR #519, item #1):
* Append cohort clause to _COMMON_WHERE so Tier S and Tier H inherit it.
* Add cohort_year_min/max binds to base_params (auto-propagates to both).
* Tier W's inline copy stays — not converted to _COMMON_WHERE (relevance_score
CASE expressions require inline form). Comment near _COMMON_WHERE documents
the dual code path requirement.
Also (reviewer item #5):
* Fix late_soviet range 1985-1999 → 1990-1999. Previous overlap with brezhnev
1970-1989 + first-match semantics made late_soviet effectively cover only
1990-1999 anyway — rename clarifies actual coverage.
Add parametrized pytest test_target_cohort_range covering None, out-of-range,
boundary cases, and the audit's target (1978 → brezhnev).
