Applied at enforcement time, not as a substitute for validation: rows saved
before this validation shipped may already hold a guard-disabling value like
0.95, and those rows must not be able to silently switch the guard off.
Non-finite input falls back to DOMINANCE_THRESHOLD_DEFAULT.
Coerce a dominance threshold into the safe range.
Applied at enforcement time, not as a substitute for validation: rows saved before this validation shipped may already hold a guard-disabling value like
0.95, and those rows must not be able to silently switch the guard off. Non-finite input falls back to DOMINANCE_THRESHOLD_DEFAULT.