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/test.sh

@@ -1,6 +1,6 @@
 #!/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
+# Requires: podman, qemu-system-x86_64, KVM, and a kernel at
 # $XDG_DATA_HOME/slim-rs/vmlinuz.
 set -u
 
@@ -102,76 +102,12 @@ EOF
     rm -rf "$work"
 }
 
-test_service() {
-    echo "=== Testing nextcloud service ==="
-
-    img="slim-test-nextcloud"
-    host_port=18080
-    work="$(mktemp -d)"
-
-    cat > "$work/Containerfile" <<'EOF'
-FROM nextcloud:32-apache
-RUN apt-get update && apt-get install -y --no-install-recommends iproute2 && rm -rf /var/lib/apt/lists/*
-EOF
-
-    echo "-- Building nextcloud container image..."
-    if ! podman build -t "$img" -f "$work/Containerfile" >/dev/null 2>&1; then
-        report fail "nextcloud service (podman build failed)"
-        rm -rf "$work"
-        return
-    fi
-
-    echo "-- Building slim VM..."
-    if ! "$SLIM_BIN" build qcow2 "$img" >/dev/null 2>&1; then
-        report fail "nextcloud service (slim build failed)"
-        cleanup "$img" "$img"
-        rm -rf "$work"
-        return
-    fi
-
-    echo "-- Booting VM with port forward (host :${host_port} -> guest :80)..."
-    setsid timeout 300 "$SLIM_BIN" run "$img" \
-        --forward "tcp:0.0.0.0:${host_port}-:80" \
-        --memory 2048M >/dev/null 2>&1 &
-    vm_pid=$!
-
-    echo "-- Waiting for Nextcloud to start..."
-    up=0
-    for _ in $(seq 1 60); do
-        if curl -s -o /dev/null -m 2 "http://localhost:${host_port}/" 2>/dev/null; then
-            up=1
-            break
-        fi
-        sleep 3
-    done
-
-    if [ "$up" = "1" ]; then
-        response=$(curl -s -L -m 10 "http://localhost:${host_port}/" 2>/dev/null || true)
-        if echo "$response" | grep -qi "nextcloud"; then
-            report pass "nextcloud service (port forward + content)"
-        else
-            report fail "nextcloud service (port reachable but no 'nextcloud' in response)"
-        fi
-    else
-        report fail "nextcloud service (port not reachable within timeout)"
-    fi
-
-    pkill -f "qemu-system-x86_64.*$img" 2>/dev/null || true
-    kill "$vm_pid" 2>/dev/null || true
-    wait "$vm_pid" 2>/dev/null || true
-
-    cleanup "$img" "$img"
-    rm -rf "$work"
-}
-
 echo "Building slim..."
 cargo build 2>&1
 
 test_distro "alpine" "alpine:latest" ""
 test_distro "archlinux" "archlinux:latest" "RUN pacman -Sy --noconfirm iputils iproute2 util-linux; pacman -Sc --noconfirm"
 
-test_service
-
 echo ""
 echo "=== Results: $pass passed, $fail failed ==="
 [ "$fail" -eq 0 ]