fix: use buffer-file-name for C single-file compile command

The fallback compile command in `cj/c-compile-command` was building paths from `(buffer-name)`. That broke for renamed buffers, uniquified names like `foo.c<2>`, and files outside `default-directory`. The buffer name is a display label, not a path, so `gcc -o name name` would compile (or fail to compile) the wrong target whenever the two diverged.

I extracted `cj/c--single-file-compile-command` that takes the source path explicitly, shell-quotes both source and output paths, and signals a clear `user-error` for non-file buffers. The fallback now passes `buffer-file-name` instead of `(buffer-name)`.

Tests for this helper landed in commit 6d58932 alongside other prog-c coverage work.

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