bot.js: the player's own slotted piece scores DIY_PIECE_BONUS (12) more; levelUp takes the
step that opens a slotted tier with copies left one round earlier (diyOpensAt). Bots field
no 自选 piece. Exports buyScoreOf / levelUp / diyOpensAt for the tests. Docs: DESIGN §6.6,
PLAYING. Golden: coop2-NORMAL-70-diy moved — the 自选 seat under AI 托管 reaches level 6 one
round earlier and fields its tier-6 Sharp at round 12.