Inject universal /slim/init instead of requiring container-specific setup

slim now works with any Containerfile by injecting distro-agnostic scripts
at build time:

- /slim/init: mounts special filesystems, configures networking (static
  QEMU slirp), sets up cgroup2, timezone, and rootless container prereqs.
  Parses slim.cmd=<base64> from the kernel cmdline for runtime overrides,
  otherwise execs /slim/exec.

- /slim/exec: generated from the image's CMD/ENTRYPOINT (via podman image
  inspect), overridable via --cmd at build and run time.

Changes:
- New src/inject.rs: inspect image config, infer command, generate
  /slim/exec script, inject /slim/ into mounted rootfs
- New src/scripts/slim-init.sh: the universal init script (include_str!)
- build.rs: rename --init to --cmd, inject scripts before packing,
  drop Meta::save, restructure to ensure unmount always runs
- qemu.rs: hardcode init=/slim/init, add --cmd (base64 on cmdline),
  drop Meta::load, add -no-reboot
- Remove src/meta.rs and meta.toml (no longer needed)
- Cargo.toml: add serde_json + base64, remove unused walkdir + cpio + toml
- Example Containerfiles simplified to plain FROM + CMD
- New example/test.sh for manual verification
- README updated for new workflow

example/archlinux/Containerfile

@@ -1,23 +1,2 @@
-# FROM archlinux:latest
-FROM marvin
-
-
-USER root
-
-# Update, install kernel, initramfs tools, and utilities
-# 'linux' includes mkinitcpio hooks that auto-generate the initramfs
-RUN pacman -Syu --noconfirm linux busybox util-linux dhcpcd; \
-    pacman -Sc --noconfirm
-
-# Copy kernel and initramfs to root for VM extraction
-RUN cp /boot/vmlinuz-linux /vmlinuz && \
-    cp /boot/initramfs-linux.img /initrd.img
-
-# Allow root login without password
-RUN passwd -d root
-
-# Set up timezone. Required to bypass archlinux's first-install setup.
-RUN ln -s /usr/share/zoneinfo/Europe/Stockholm /etc/localtime
-
-COPY init /init
-RUN chmod +x /init
+FROM archlinux:latest
+CMD ["/bin/sh"]