In-flight coalescing already dedups identical misses, but acceptance
still lacked a gated test (without a hold, waiters can become sequential
HITs after the first fill) and the Quick check metrics table omitted
cache_coalesced.
Add steamcache/coalesce_test.go: N concurrent identical GETs share one
upstream fill (leader MISS, waiters HIT-COALESCED) and a 5xx sibling.
Document cache_coalesced next to the other hit/miss fields.
Fixes#35
