Pure required-field validation used by handleAction("next"). Extracted from BlockBasedStepView's canAdvance walk so the advance gate is unit-testable without a live host, proving an interaction-driven advance can NOT bypass validation. Returns the first missing block's label (for the validation pill copy) or null. Mirrors iOS RequiredFieldGate (Android keeps the extra List<*> branch so an empty multi-select still fails the gate — pre-existing behavior).