damocles suggested using lib.types.strMatching for the target option
itself rather than relying solely on the post-hoc assertion. Pattern:
`^[A-Za-z0-9_][A-Za-z0-9_./-]*$` — first char alphanumeric/_, then
alphanumerics + _ + . + / + - allowed (so nested layouts like
"games/bitburner" still work).
This rejects at type-check time:
- leading `/` (absolute paths)
- leading `.` (so `..` as a full string blocked, also `./foo`)
- leading `-` (would parse as flag by some tools)
- spaces, control chars, weird unicode
The existing assertion stays — it catches mid-path `..` segments
(`foo/../bar`) that the regex can't reject without lookahead. POSIX
regex (which nix uses) doesn't support lookahead, so the
type-and-assertion split is the cleanest expression.
Refs #273.
65532e8387