fix: shell-quote F6 test-runner command arguments

`cj/--f6-test-runner-cmd-for` was building shell command strings with raw paths and stems via `format`. For ordinary names (`tests/test_foo.py`, `pkg/foo`) that worked fine. But a path with spaces or a stem with shell metacharacters would break or misbehave once the string hit `compile`. A Python test file under `dir with spaces/` would get tokenized as separate arguments.

I added `cj/--f6-shell-quote-argument` that escapes only when the argument doesn't match `cj/--f6-shell-safe-argument-regexp` (alphanumerics, slash, dot, dash, plus a small handful of safe punctuation). Ordinary paths skip the quoter and stay readable. Risky paths route through `shell-quote-argument`.

I wrapped the four interpolations in the test-runner builder: the elisp `FILE=` basename, the elisp `TEST=^test-stem-` regex, both pytest paths, and the Go `./rel-dir`. The Go branch also handles an empty rel-dir explicitly so the result stays `go test ./` instead of constructing `./` via format with an empty string.

I added three boundary tests: a Python path with spaces, an elisp stem with `;`, and a Go directory with spaces. Existing tests for ordinary paths continue to pass since the safe regex covers them.

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