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/alpine/Containerfile

@@ -1,9 +1,2 @@
 FROM alpine:latest
-RUN mkdir -p /lib/apk/db /run
-RUN apk add --no-cache --initdb linux-virt busybox util-linux
-RUN cp /boot/vmlinuz-virt /vmlinuz && echo "Welcome to slim!" > /etc/motd
-
-# Use custom init script
-COPY init /init
-RUN chmod +x /init
-
+CMD ["/bin/sh"]

example/alpine/init

@@ -1,29 +0,0 @@
-#!/bin/sh
-
-# Create console device
-# TODO: is this necessary?
-[ -c /dev/console ] || mknod -m 600 /dev/console c 5 1
-
-# Mount special filesystems
-mkdir -p /proc /sys
-mount -t proc proc /proc
-mount -t sysfs sysfs /sys
-
-# Networking
-echo slim > /proc/sys/kernel/hostname
-ip link set lo up 2>/dev/null
-ip link set eth0 up 2>/dev/null
-udhcpc -i eth0 -f -q # Get DHCP lease
-
-echo
-echo 'Welcome to slim!'
-echo
-
-# Run interactive shell
-/bin/sh </dev/console >/dev/console 2>&1
-
-poweroff -f
-while true; do
-    sleep 1
-    poweroff -f
-done