• Compute a Boost Mystery's mastery-level contribution to its associated skill, following the Mastery Boost algorithm:

    • Existing skill: all N boosts compound off the skill's seeded mastery level; the contribution is the resulting EML delta.
    • Absent skill: the first boost opens the skill at its Skill Base and the remaining N − 1 compound off that; the skill is conferred at the result.

    The computation is derived state — a caller re-runs it every prepare cycle and contributes the result as a live delta, so the effect reverts automatically when the Mystery lapses.

    Returns BoostContribution

    The contribution; see BoostContribution.