Compute the in-file stream index for external subtitles instead of
hardcoding -map 1:0. For single-stream files (SRT/ASS/VTT) the index
is always 0, preserving existing behavior. For multi-stream containers
like MKS, the correct track is selected by counting sibling streams
that share the same Path.
Add unit tests for GetMapArgs covering internal subs, external SRT,
multiple external files, and multi-stream MKS containers.
a15b426e73