Function mergeMilestones

mergeMilestones(
    existing: Record<string, TiltifyMilestone[]>,
    incoming: Record<string, TiltifyMilestone[]> | undefined,
    authoritative?: boolean,
): Record<string, TiltifyMilestone[]>
Accumulates milestones across ticks, keyed by campaign ID then milestone ID.
Parameters
- existing: Record<string, TiltifyMilestone[]>
  Current per-campaign milestone map.
- incoming: Record<string, TiltifyMilestone[]> | undefined
  Fresh per-campaign milestone map (may be undefined).
- authoritative: boolean = false
  When true, replace the named campaigns wholesale instead of merging. Defaults to false.
Returns Record<string, TiltifyMilestone[]>
Merged map with milestones preserved across ticks.
- Defined in realtime-pipeline/src/reducer/tiltifyData.ts:158