Add B² Network mainnet

[a10zn8]

Summary

• Adds B² Network mainnet protocol entry (chain-id 0xdf / 223, grpcId 1158) to chains.yaml.
• Adds matching chains-meta.yaml metadata (BTC, decimals 18, explorer https://explorer.bsquared.network).

B² Network is an OP Stack Bitcoin L2 — uses ethereum-pos connector type, expected block time 2s.

Companion PRs

• emerald-grpc: Add CHAIN_BSQUARED__MAINNET constant emerald-grpc#155
• public: Add B² Network mainnet #218
• dproxy: https://github.com/drpcorg/dproxy/pull/2547
• dshackle: Add B² Network mainnet support dshackle#826
• nodecore: Add B² Network mainnet support nodecore#180
• infra: https://github.com/drpcorg/infra/pull/4644

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