Adapt T384549: Create a per-user flag for enabling SUL3 for signup. We can set the opt-in flag once the user account got created, to make subsequent login flows consistent with the signup flow, but for a signup we need to decide on the fly. Probably what we want is to adjust the isSul3Enabled() check in SpecialPageBeforeExecuteHookHandler::onSpecialPageBeforeExecute() (responsible for deciding which flow to use): if the user is not logged in, and isn't actively opted in or opted out, use a site configuration flag to determine with what probability the user ends up in SUL3.
This should still be overridable (there will be an opt-out link per T377140: Adapt user experience to expose the progressive rollout with a fallback link leading to the old signup page, and so the usesul3=0 flag needs to reliably work there). Probably easiest to do as a new $wgCentralAuthEnableSul3 type that takes lower priority than most other options; but unlike the always option, it must be specific to signup. Or maybe we should just split $wgCentralAuthEnableSul3 into a login and a signup version.