ethrpc: add robinhood chains to the Networks registry

Adds robinhood (4663) and robinhood-testnet (46630) to ethrpc.Networks,
which ethreceipts consults for NumBlocksToFinality when a caller does
not set its own.

NumBlocksToFinality 50 matches every arbitrum entry in this map
(arbitrum, arbitrum-testnet, arbitrum-nova), which is the right
reference since robinhood is an Arbitrum Orbit chain. Without an entry
both chains fell back to DefaultNumBlocksToFinality of 40 -- workable,
but implicit and inconsistent with the rest of the family.

Note 50 blocks is ~5s at robinhood's 100ms block time, against ~12.5s
for arbitrum at 250ms. Reorg depth is block-based, so matching the
family's depth is the sensible reading, but the wall-clock window is
correspondingly shorter.

There is no ArbitrumChain field on the Network struct -- only
OptimismChain -- so nothing else to set here.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: pkieltyka

ethrpc/networks.go

@@ -154,4 +154,14 @@ var Networks = map[uint64]Network{
 		NumBlocksToFinality: 50,
 		OptimismChain:       true,
 	},
+	4663: {
+		Name:                "robinhood",
+		ChainID:             4663,
+		NumBlocksToFinality: 50,
+	},
+	46630: {
+		Name:                "robinhood-testnet",
+		ChainID:             46630,
+		NumBlocksToFinality: 50,
+	},
 }