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

Cargo.toml

@@ -5,14 +5,13 @@ edition = "2024"
 
 [dependencies]
 anyhow = "1.0.104"
+base64 = "0.22"
 clap = { version = "4.6.6", features = ["derive"] }
 flate2 = "1.1.9"
-walkdir = "2.5.0"
-cpio = "0.4.1"
+serde = { version = "1.0.229", features = ["derive"] }
+serde_json = "1.0"
 xdg = "3.0.0"
 tempfile = "3.27.0"
-toml = "1.1.4"
-serde = { version = "1.0.229", features = ["derive"] }
 
 [dev-dependencies]
 tempfile = "3.27.0"