Function formatHandleId

formatHandleId(path: string, subsong?: number): string
Build a canonical handle-id from a path + optional subsong index.
- Returns '' for falsy / non-string path.
- Subsong 0 (or non-numeric) collapses to the bare path; only strictly positive sub-song indices receive the |subsong:N tail.
Parameters
- path: string
- subsong: number = 0
Returns string
Example
```
formatHandleId('C:\\song.flac', 2) === 'C:\\song.flac|subsong:2'
  formatHandleId('C:\\song.flac')    === 'C:\\song.flac'
  formatHandleId('')                 === ''
```
- Defined in sdk/src/smp/handleId.ts:27