pds check <file> fails with `walking ``: IO error` — directory form works

[starscream-bot]

Version: pds 0.1.7 (linux x86_64)

Repro — fresh workspace, exactly as pds init suggests:

$ mkdir scratch && cd scratch && pds init
  created ./pds.toml
  created ./main.pds
Initialized PseudoScript workspace in .
Next: `pds check main.pds`, then `pds doc --serve`.

$ pds check main.pds
error: : walking ``: IO error for operation on : No such file or directory (os error 2): No such file or directory (os error 2)
$ echo $?
1

$ pds check .
$ echo $?
0

Expected: pds check main.pds checks the file (it is the command pds init itself recommends).

Actual: exits 1 with a walker IO error on an empty path. Any .pds file argument reproduces it; only the directory form works.

Looks like the workspace walker is handed the file's parent-path component (empty string for a bare filename) instead of the resolved workspace root. Guess: pds check sub/file.pds walking sub/ and missing pds.toml would be the same shape.

Workaround: pds check . / pds check <dir>.