UPGRADE_SOURCES nonempty tuple

Trap: typing UPGRADE_SOURCES as readonly UpgradeSource[] looks sufficient, but noUncheckedIndexedAccess makes UPGRADE_SOURCES[0] possibly undefined and contaminates every default source parameter. Fix: declare it with as const satisfies readonly [UpgradeSource, ...UpgradeSource[]]; this proves the ordered fallback list is nonempty while validating every element.