Add CLI option parsing and implement layer unmounting

Replace the implicit single-argument invocation with getopt_long-based
flags: -m/--mount (existing mount flow, now explicit), -u/--umount
(unmounts a layer via containers-storage), -t/--test (stub),
-l/--log-level (runtime spdlog level), -h/--help, -V/--version.
--mount now also prints the top layer's ID so it can be passed to
--umount.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

meson.build

@@ -26,4 +26,4 @@ fixture_tar = custom_target('oci-fixture',
     output : 'fixture.tar',
     command : [find_program('python3'), files('tests/gen_fixture.py'), '@OUTPUT@'])
 
-test('test', slocker_lite, args : [fixture_tar])
+test('test', slocker_lite, args : ['-m', fixture_tar])