Function buildBranchSegment

buildBranchSegment(parentStageId: string, index: number): string
Build the generated subflow path segment for one parallelForEach branch.
Parameters
- parentStageId: string
  The fan-out stage's id (already subflow-prefixed when the chart is mounted inside another chart, so nested composition works without special handling).
- index: number
  The branch's position in the items array (0-based).
Returns string
Example
```
buildBranchSegment('review-chunks', 2)        // 'review-chunks~2'
buildBranchSegment('outer/review-chunks', 2)  // 'outer/review-chunks~2'
```
- Defined in lib/ids/branchSegment.ts:74