Implement ext4l_exists() to check if a file or directory exists on the
filesystem. This uses ext4l_resolve_path() to look up the path and
returns 1 if found, 0 otherwise.
Wire the function into fs_legacy.c and add a basic test.
Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>
0e9c88b170