Function toHandleId

Coerce a loose handle-like value to its canonical handle-id string.

Accepts:

• bare strings (returned as-is, including any |subsong:N suffix);
• FbMetadbHandle-shaped objects (via HandleId getter);
• track-info objects with Path + optional SubSong;
• track-info objects with absolutePath / path + optional subsong, the shape of a host track row. A path that already ends in |subsong:N keeps that suffix and subsong is not applied again.

Returns '' when no path-like field is present.