Boolean-literal mutation: true ↔ false.
Only literals in runtime positions are mutated, never pattern literals. A flip that would reproduce the original value is impossible, so every eligible boolean yields exactly one mutant.
This is the boolean flip that used to be the second arm of the old literal family (whose integer
arm is now Mutare.Mutators.IntegerLiteral). It owns true/false the way
Mutare.Mutators.AtomLiteral defers them here rather than treating them as ordinary atoms. Distinct
from Mutare.Mutators.Conditional, which forces boolean-valued operator expressions (a > b,
a and b) to a constant — it never fires on a bare literal.
Filterable variants — qualify a # mutare:ignore filter with :label to suppress the one kind
(Mutare.Mutator.variants/0): negate.