Drop -m/-u/-c short options for --mount/--umount/--cleanup

Now that -r/--run and -x/--exec cover normal use, --mount/--umount/--cleanup
are debug-only escape hatches not worth a short letter. Reassigned their
long_options codes to long-option-only constants (options::mount/umount/
cleanup) and dropped m:/u:/c: from getopt_long's own short-options string.

Updated the fixture smoke test (tests/run_test.py) and docs, which invoked
-m/-u/-c directly.

CLAUDE.md

@@ -329,8 +329,9 @@ Source layout (all under `src/`):
   compiled into this build" message and returns nonzero instead of
   failing to link. `-t`'s own leftover-args capture requires a literal
   `--` before any Catch2 option that looks like one of slocker-lite's own
-  (`-r/--reporter` collides with `-r/--run`, `-c/--section` with
-  `-c/--cleanup`) — a bare tag expression like `-t -- "[unit]"` needs it
+  (`-r/--reporter` collides with `-r/--run` — `-c/--section` no longer
+  collides with anything now that `--cleanup` dropped its own `-c` short
+  form) — a bare tag expression like `-t -- "[unit]"` needs it
   too by convention, though `getopt_long`'s own permutation happens to let
   a `-t "[unit]"` without `--` work anyway, since `"[unit]"` doesn't start
   with `-`. Distinct from the Meson-driven fixture smoke test under
@@ -2277,8 +2278,8 @@ Build directory is `buildDir/` (already configured).
 - Build: `meson compile -C buildDir` (or `ninja -C buildDir`) — also builds
   `buildDir/slocker-lite-priv-drop`, the statically-linked helper `-r --user` needs
   (see `priv_drop_helper.cpp` in "Project state")
-- Run the executable: `./buildDir/slocker-lite -m <image.tar>` (see `--help` for the
-  full flag list: `-m/--mount`, `-r/--run`, `-u/--umount`, `-c/--cleanup`,
+- Run the executable: `./buildDir/slocker-lite --mount <image.tar>` (see `--help` for the
+  full flag list: `--mount`, `-r/--run`, `--umount`, `--cleanup`,
   `-l/--list-images`, `-i/--inspect`, `-x/--exec`, `--kill`, `--no-nsenter`, `-D/--daemonize`,
   `--user`, `--group`, `--hostname`, `--env`, `--env-file`, `-v/--volume`, `--list-volumes`, `--delete-volume`,
   `--delete-volume-full`, `-n/--network`, `--extern`, `--intern`, `--subnet`, `--with-ipv6`, `--subnet6`,