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/main.rs

@@ -2,7 +2,7 @@ mod build;
 mod command;
 mod forward;
 mod image;
-mod meta;
+mod inject;
 mod qemu;
 mod registry;
 
@@ -12,7 +12,7 @@ use clap::{Parser, Subcommand};
 use crate::{build::BuildCmd, image::ImageCmd, qemu::RunCmd};
 
 #[derive(Parser, Debug)]
-#[command(about = "Build bootable initrd VMs from container images")]
+#[command(about = "Turn containers into bootable VMs")]
 struct Cli {
     #[command(subcommand)]
     command: Commands,