The actor documents to push the effect to
UUID of the effect/stackable-effect to apply (ActiveEffect or journal page)
Initial stack count for stackable effects (ignored for non-stackable)
The party actor document that will hold the effect records
Map of memberId → {effect, linkItem}
Push a single effect to all party members with a given initial stack count. Creates one ActiveEffect record per member on the party, then creates a partyEffectLink pointer item on each member's actor.
Uses a mutex scoped to the party document to prevent concurrent push/un-push races.