Robinhood Chain: hub entry + 3 bench integrations

[Flotapponnier]

Summary

• /chains/robinhood: CHAINS entry (page already had bench coverage via aggregator-head-lag measuring a Robinhood pool) + chain-kpis registry entry (DefiLlama "Robinhood Chain" verified, Mobula "Robinhood Chain" verified with key, ETH native)
• l2-block-time: new HTTP polling mode (Poll flag) for sequencers with no public WSS. Robinhood's block time is derived from eth_blockNumber deltas over 2s windows, N samples of elapsed/N per window preserves per-block weighting; methodology discloses the jitter limitation. Verified live: ~100ms blocks, the chain becomes the fastest of the cohort
• network-fees: 15th chain (eth_feeHistory verified, base fee ~0.05 gwei, zero priority rewards, Orbit pattern), title/seo counts bumped
• All endpoints verified by probe before inclusion (chain id 0x1237, official RPC keyless)