Optional ReadonlybranchesA parallelForEach stage id → the chart one branch runs.
Optional ReadonlylazyA lazy mount's subflow id → the chart it resolves to.
Optional ReadonlyloopsA loop's TARGET id (what loopTo names) → the most passes through it each time the chart reaches it (≥ 1).
What the caller knows that the chart does not declare. Keys are ids as the run names them.