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

src/qemu.rs

@@ -1,6 +1,6 @@
 //! QEMU: boot a registry entry with qemu-system-x86_64 using direct kernel
 //! boot (-kernel/-initrd or -drive). The kernel is shared across all VMs
-//! at $XDG_DATA_HOME/slim-rs/vmlinuz. A universal /slim/init script (injected
+//! at $XDG_DATA_HOME/slim-rs/registry/vmlinuz. A universal /slim/init script (injected
 //! at build time) handles VM bootstrap; /slim/exec runs the container's
 //! CMD/ENTRYPOINT. Runtime overrides are delivered as `slim.cmd=<base64>`
 //! on the kernel cmdline.