true when the boost confers an absent skill (the working ML is the
level the skill is granted at); false when it augments an existing skill
(the working ML feeds BoostContribution.delta).
The EML contribution. For an existing skill it is the delta over the
seeded base (workingML - seedML); for a conferred skill it is the full
workingML (the skill has no prior EML). 0 when count ≤ 0.
The mastery level after all boosts were applied.
Result of computeBoostContribution.