Bridge v2: enable the fast tier (Arbitrum Sepolia, btUSD)

Enables the stake-weighted fast tier of bridge v2 for SunimaBridge v9 0x1721dBF0E5A13Dad966D56f988ad7aa04664Ff98 on Arbitrum Sepolia (421614), code hash 0xe2e57ebc...f98b, deposits slot 14. One token: btUSD test token 0x90da213d6af490c7BC22C271152a268E99F9aa6b (contract caps 10,000 per deposit / 1,000,000 total; Sunima fast caps equal). Quorum 67% of bonded power, max 33.34% per voter, min 4 attesters, attest stride 2400 L2 blocks, max gap 57,600, release valve on (testnet). Starting L2 height 322,780,800 (projected Arbitrum height at execution). Validators attest with sunimad bridge-watch.