chore: kill fuji — no Avalanche/Fuji terminology (#49)

Per canonical rule: Lux is its own thing, fuji isn't ours. Sweep
removes positive references to the Avalanche testnet name from
comments, function names, test fixtures, and example data. Where
defensive code rejects non-Lux HRPs (lux/genesis), the test input
swapped from 'P-fuji1...' to 'P-avax1...' so the rejection behavior
still proves out without naming fuji as a thing.

Build clean.

engines/avalanche/engine.go

@@ -247,11 +247,11 @@ func getLuxNetwork(networkID uint32) string {
 	case 1:
 		return "mainnet"
 	case 5:
-		return "fuji"
+		return "testnet"
 	case 43114:
 		return "mainnet" // C-Chain ID
 	case 43113:
-		return "fuji" // Fuji C-Chain
+		return "testnet" // testnet C-Chain
 	default:
 		return fmt.Sprintf("%d", networkID)
 	}