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

README.md

@@ -28,5 +28,5 @@ podman image rm slim-alpine
 
 ## Kernel
 
-slim boots VMs using a shared kernel at `$XDG_DATA_HOME/slim-rs/registry/vmlinuz`.
+slim boots VMs using a shared kernel at `$XDG_DATA_HOME/slim-rs/vmlinuz`.
 Place a suitable x86 bzImage there before running `slim run`.

example/test.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 # Test script for slim - boots containers and verifies CMD inference/override.
 # Requires: podman, qemu-system-x86_64, KVM, curl, and a kernel at
-# $XDG_DATA_HOME/slim-rs/registry/vmlinuz.
+# $XDG_DATA_HOME/slim-rs/vmlinuz.
 set -u
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"