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

@@ -0,0 +1,113 @@
+#!/bin/bash
+# Test script for slim - boots containers and verifies CMD inference/override.
+# Requires: podman, qemu-system-x86_64, KVM, and a kernel at
+# $XDG_DATA_HOME/slim-rs/vmlinuz.
+set -u
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+SLIM_BIN="$SCRIPT_DIR/../target/debug/slim"
+TIMEOUT=120
+
+pass=0
+fail=0
+
+report() {
+    if [ "$1" = "pass" ]; then
+        echo "  PASS: $2"
+        pass=$((pass + 1))
+    else
+        echo "  FAIL: $2"
+        fail=$((fail + 1))
+    fi
+}
+
+check_output() {
+    output="$1"
+    marker="$2"
+    label="$3"
+    if echo "$output" | grep -q "$marker"; then
+        report pass "$label"
+    else
+        report fail "$label (expected marker: $marker)"
+    fi
+}
+
+cleanup() {
+    "$SLIM_BIN" image rm "$1" >/dev/null 2>&1 || true
+    podman image rm "$2" >/dev/null 2>&1 || true
+}
+
+test_distro() {
+    distro="$1"
+    from="$2"
+    extra_setup="$3"
+
+    echo "=== Testing $distro ==="
+
+    work="$(mktemp -d)"
+
+    # === Test 1: CMD-inferred ===
+    echo "-- Test 1: CMD-inferred"
+    cat > "$work/Containerfile.infer" <<EOF
+FROM $from
+$extra_setup
+CMD ["/bin/sh", "-c", "echo CMD_INFERRED_OK; ping -c1 1.1.1.1 >/dev/null 2>&1 && echo PING_OK; ping -c1 example.org >/dev/null 2>&1 && echo DNS_OK; poweroff -f"]
+EOF
+    img="slim-test-$distro-infer"
+    if podman build -t "$img" -f "$work/Containerfile.infer" >/dev/null 2>&1; then
+        "$SLIM_BIN" build qcow2 "$img" >/dev/null 2>&1
+        output=$(timeout "$TIMEOUT" "$SLIM_BIN" run "$img" 2>&1 || true)
+        check_output "$output" "CMD_INFERRED_OK" "$distro CMD-inferred"
+        check_output "$output" "PING_OK" "$distro ping 1.1.1.1"
+        check_output "$output" "DNS_OK" "$distro DNS lookup"
+    else
+        report fail "$distro CMD-inferred (podman build failed)"
+    fi
+    cleanup "$img" "$img"
+
+    # === Test 2: slim run --cmd override ===
+    echo "-- Test 2: slim run --cmd override"
+    cat > "$work/Containerfile.run" <<EOF
+FROM $from
+$extra_setup
+CMD ["/bin/sh", "-c", "echo SHOULD_NOT_APPEAR; poweroff -f"]
+EOF
+    img="slim-test-$distro-run"
+    if podman build -t "$img" -f "$work/Containerfile.run" >/dev/null 2>&1; then
+        "$SLIM_BIN" build qcow2 "$img" >/dev/null 2>&1
+        output=$(timeout "$TIMEOUT" "$SLIM_BIN" run "$img" --cmd 'echo RUN_OVERRIDE_OK; poweroff -f' 2>&1 || true)
+        check_output "$output" "RUN_OVERRIDE_OK" "$distro run --cmd override"
+    else
+        report fail "$distro run --cmd override (podman build failed)"
+    fi
+    cleanup "$img" "$img"
+
+    # === Test 3: slim build --cmd override ===
+    echo "-- Test 3: slim build --cmd override"
+    cat > "$work/Containerfile.build" <<EOF
+FROM $from
+$extra_setup
+CMD ["/bin/sh", "-c", "echo SHOULD_NOT_APPEAR; poweroff -f"]
+EOF
+    img="slim-test-$distro-build"
+    if podman build -t "$img" -f "$work/Containerfile.build" >/dev/null 2>&1; then
+        "$SLIM_BIN" build qcow2 "$img" --cmd 'echo BUILD_OVERRIDE_OK; poweroff -f' >/dev/null 2>&1
+        output=$(timeout "$TIMEOUT" "$SLIM_BIN" run "$img" 2>&1 || true)
+        check_output "$output" "BUILD_OVERRIDE_OK" "$distro build --cmd override"
+    else
+        report fail "$distro build --cmd override (podman build failed)"
+    fi
+    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"
+
+echo ""
+echo "=== Results: $pass passed, $fail failed ==="
+[ "$fail" -eq 0 ]