fix: handle ordinary directories when copying with -P - #1269

With -P, cp currently skips directory handling even when the source is an ordinary directory. Both cp -P source dest and cp -RP source dest then try to read that directory as a file and throw EISDIR.

Use lstat when preserving symlinks so ordinary directories still reach the existing directory handling: omit them without -R, or copy them recursively with -R. Actual symlinks remain preserved, and -L keeps its existing precedence.

Adds regressions for nonrecursive directory omission, recursive directory copying, and preserving a symlink inside a copied directory. All three fail on the original code.

Validation: npm test passes on macOS and Linux (627 passed, 6 skipped, including the posttest ESLint check); npm run check-node-support passes on Linux.

Fixes #937.