fix(matchmaking): re-enqueue opens a new game, not the caller's own

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.

backend/internal/game/service.go

@@ -266,10 +266,11 @@ func (svc *Service) Create(ctx context.Context, params CreateParams) (Game, erro
 
 // OpenOrJoin enters accountID into auto-match for the variant and per-turn rule in
 // params and returns the game they land in immediately: another waiting player's open
-// game (joined=true), the caller's own still-open game on a re-enqueue, or a fresh open
-// game seating only the caller with an empty opponent seat that a human or the reaper's
-// robot fills later. openDeadline is when the reaper substitutes a robot into a freshly
-// opened game (ignored when joining one). The bag seed defaults to random; params.Seed
+// game (joined=true), or a fresh open game seating only the caller with an empty
+// opponent seat that a human or the reaper's robot fills later. A re-enqueue while the
+// caller is already waiting opens another game rather than returning their own.
+// openDeadline is when the reaper substitutes a robot into a freshly opened game
+// (ignored when joining one). The bag seed defaults to random; params.Seed
 // pins it. First-move fairness comes from seating the caller at seat 0 or seat 1
 // (derived from the seed): seated at seat 1, the still-empty seat 0 moves first, so the
 // caller just waits for the opponent. It backs the lobby auto-match enqueue.