Solo pollers park (3001 RETRY); two concurrent pollers pair and both
receive 3004 + same BattleId. Cache hits on the first arriver's next
poll. ?scripted=1 retains today's solo Scripted path for dev work.
Response DTO's BattleId/NodeServerUrl become nullable so 3001 omits
them on the wire (WhenWritingNull policy drops them).
ASP.NET's default bool binder rejects "1" as a value, so the scripted
opt-in is bound as string? and parsed permissively (accepts "1" and
"true"/"True"/etc.) rather than relying on built-in bool binding.
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