Publish moved the skel and AppStream emission out of the per-component
loop, and codecov flags the relocated error branches as uncovered. Cover
them with deterministic failures:
- a regular file in the middle of the skel path makes the walk fail
with ENOTDIR, which is not swallowed as a not-exist error
- nested skel/AppStream names each within NAME_MAX flatten into a
temporary index file name that exceeds it, failing index creation
- an unreadable skel file (mode 0) fails the open (non-root only)
- a ~1MB filesystem as TMPDIR runs out of space mid-write, reusing the
/smallfs convention from DiskFullSuite
Also add a unit-level happy path test publishing nested skel files and
checking they land in the published tree and the Release checksums.
KereMath
c54d44e070