refactor(dirvish): extract cj/--ediff-pair-from-files; fix 0-files crash

`cj/dired-ediff-files' had its pair-determination logic inline: count check, prompt fallback when only one file was marked, and the older-first ordering for `ediff-files'.  Lift it into `cj/--ediff-pair-from-files' -- pure given the file list, an injected prompt thunk, and a newer-than-p comparator -- so tests stay independent of mtimes and the dired prompt.

While extracting, surface a latent bug: with zero marked files the original code fell through to `(file-newer-than-file-p nil nil)' and crashed with a wrong-type-argument error.  Replace the crash with a clear `user-error' ("No files marked"), and add a regression test.  The 3+ files case keeps its existing user-error message.

Five Normal/Boundary/Error tests cover both ordering directions, the one-file prompt path, and both error counts.

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