POSIX-compliant

I can't generate a gif from my tape without switching to a bloated shell.
When I run:
`vhs cassette.tape`
I get:
`File: cassette.tape`
`failed to execute command: invalid shell mksh`
`recording failed`

Start by reproducing the failure with `vhs cassette.tape` under `mksh` and trace the command path that reports `invalid shell mksh`. No source file or test is named in the issue, so locate the shell validation and recording entry points first. Done means the cassette records a GIF under a POSIX-compatible shell without requiring a different shell.

Written by the indexing model from the issue text.