Function parseBranchSegment

parseBranchSegment(
segment: string,
): { index: number; parentStageId: string } | undefined
Decompose a generated branch segment back into its parts, or undefined when segment is not one.

Splitting at the LAST marker is unambiguous precisely because the builder refuses the marker in the parent id (see the module doc) — no right-most numeric-scanning heuristic is needed, and the round trip parseBranchSegment(buildBranchSegment(id, i)) is exact.
Parameters
- segment: string
Returns { index: number; parentStageId: string } | undefined
Example
```
parseBranchSegment('review-chunks~2')  // { parentStageId: 'review-chunks', index: 2 }
parseBranchSegment('sf-tools')         // undefined
```
- Defined in lib/ids/branchSegment.ts:104