Support -F (--classify) option in ls (#117)

* Support -F (--classify) option in ls (#112)

Appends type indicators to entry names: / for directories, * for
executables, @ for symlinks. Works with all existing flags (-l, -a,
-R, -d, etc.). Uses lstat() for correct symlink detection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: use stat() for entry type, lstat() only for classify suffix

Addresses review feedback: the unconditional stat() → lstat() change
caused symlinks to directories to appear as regular files. Now stat()
is used for mode/size/type determination (follows symlinks), and
lstat() is only called when -F/--classify needs symlink detection.

Adds tests for symlink-to-directory with -F and -lF flags.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>