A second "random opponent" enqueue with the same variant and per-turn
rule, while the caller's first game was still open (awaiting an
opponent), returned that same open game, so a player could never start a
fresh random game while one was still searching.
Drop the own-open short-circuit (step 1) in store.OpenOrJoin: a
re-enqueue now joins another player's open game or opens a fresh one.
Accumulation stays bounded by MaxActiveQuickGames, which counts open
games. Update the matchmaker/service/store doc comments and
ARCHITECTURE.md, and flip the pinning test to assert the new behavior.
Ilia Denisov