Any FormulaError, possibly wrapped in any
number of propagated layers.
The deepest non-propagated variant reachable through
cause. If error is itself non-propagated, it is returned
unchanged.
Diagnostics — error reporters, devtool overlays, tests — frequently want to know where a failure actually started, not the relay hop that surfaced it. The walk is bounded by the depth of the dependency chain that produced the error, which is itself bounded by the cycle detector, so unbounded recursion is not representable here.
Walk a FormulaError chain and return the originating non-
propagatedvariant.