Reduce nested loops in subgraph mappings

[rickstaa]

Some subgraph mappings use nested loops (e.g. roundsManager.ts broadcaster tracking). While functional today, nested iteration in mappings can become expensive as data grows and may impact indexing performance:

• subgraph/src/mappings/roundsManager.ts

  Line 158 in 4728c85

  for (let i = 0; i < daysLength; i++) {

• subgraph/src/mappings/roundsManager.ts

  Line 199 in 4728c85

  for (let j = 0; j < broadcasterDaysLength; j++) {

Goal: Refactor mapping logic to avoid nested loops by using more efficient data structures or indexing strategies.