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.lock

@@ -64,6 +64,12 @@ version = "1.0.104"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "330a5ed07fa54e4702c9d6c4174f74427fc0ef6e214bbd677ae50a5099946470"
 
+[[package]]
+name = "base64"
+version = "0.22.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72b3254f16251a8381aa12e40e3c4d2f0199f8c6508fbecb9d91f575e0fbb8c6"
+
 [[package]]
 name = "bitflags"
 version = "2.13.1"
@@ -122,12 +128,6 @@ version = "1.0.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1d07550c9036bf2ae0c684c4297d503f838287c83c53686d05370d0e139ae570"
 
-[[package]]
-name = "cpio"
-version = "0.4.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "938e716cb1ade5d6c8f959c13a7248b889c07491fc7e41167c3afe20f8f0de1e"
-
 [[package]]
 name = "crc32fast"
 version = "1.5.1"
@@ -137,12 +137,6 @@ dependencies = [
  "cfg-if",
 ]
 
-[[package]]
-name = "equivalent"
-version = "1.0.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "877a4ace8713b0bcf2a4e7eec82529c029f1d0619886d18145fea96c3ffe5c0f"
-
 [[package]]
 name = "errno"
 version = "0.3.14"
@@ -180,34 +174,24 @@ dependencies = [
  "r-efi",
 ]
 
-[[package]]
-name = "hashbrown"
-version = "0.17.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ed5909b6e89a2db4456e54cd5f673791d7eca6732202bbf2a9cc504fe2f9b84a"
-
 [[package]]
 name = "heck"
 version = "0.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
 
-[[package]]
-name = "indexmap"
-version = "2.14.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "07aa2048142242915a31d35844fb311e0e53fcca590c3a0a40dcf1b841fa09eb"
-dependencies = [
- "equivalent",
- "hashbrown",
-]
-
 [[package]]
 name = "is_terminal_polyfill"
 version = "1.70.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a6cb138bb79a146c1bd460005623e142ef0181e3d0219cb493e02f7d08a35695"
 
+[[package]]
+name = "itoa"
+version = "1.0.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8f42a60cbdf9a97f5d2305f08a87dc4e09308d1276d28c869c684d7777685682"
+
 [[package]]
 name = "libc"
 version = "0.2.189"
@@ -220,6 +204,12 @@ version = "0.12.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "32a66949e030da00e8c7d4434b251670a91556f4144941d37452769c25d58a53"
 
+[[package]]
+name = "memchr"
+version = "2.8.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cf8baf1c55e62ffcace7a9f06f4bd9cd3f0c4beb022d3b367256b91b87513d98"
+
 [[package]]
 name = "miniz_oxide"
 version = "0.8.9"
@@ -279,15 +269,6 @@ dependencies = [
  "windows-sys",
 ]
 
-[[package]]
-name = "same-file"
-version = "1.0.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "93fc1dc3aaa9bfed95e02e6eadabb4baf7e3078b0bd1b4d7b6b0b68378900502"
-dependencies = [
- "winapi-util",
-]
-
 [[package]]
 name = "serde"
 version = "1.0.229"
@@ -319,12 +300,16 @@ dependencies = [
 ]
 
 [[package]]
-name = "serde_spanned"
-version = "1.1.1"
+name = "serde_json"
+version = "1.0.151"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6662b5879511e06e8999a8a235d848113e942c9124f211511b16466ee2995f26"
+checksum = "c841b55ecdae098c80dcae9cf767f6f8a0c2cdb3416bbef72181df4d0fe73f14"
 dependencies = [
+ "itoa",
+ "memchr",
+ "serde",
  "serde_core",
+ "zmij",
 ]
 
 [[package]]
@@ -338,13 +323,12 @@ name = "slim"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "base64",
  "clap",
- "cpio",
  "flate2",
  "serde",
+ "serde_json",
  "tempfile",
- "toml",
- "walkdir",
  "xdg",
 ]
 
@@ -378,45 +362,6 @@ dependencies = [
  "windows-sys",
 ]
 
-[[package]]
-name = "toml"
-version = "1.1.4+spec-1.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3aace63f4bbcdfc2c965b059de67119c89c4017a70d633be6c104910f67056f5"
-dependencies = [
- "indexmap",
- "serde_core",
- "serde_spanned",
- "toml_datetime",
- "toml_parser",
- "toml_writer",
- "winnow",
-]
-
-[[package]]
-name = "toml_datetime"
-version = "1.1.1+spec-1.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3165f65f62e28e0115a00b2ebdd37eb6f3b641855f9d636d3cd4103767159ad7"
-dependencies = [
- "serde_core",
-]
-
-[[package]]
-name = "toml_parser"
-version = "1.1.3+spec-1.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1d38ac1cf9b95face32296c0a3ede1fdc270627c9d9c02a7274dd6d960dc4d56"
-dependencies = [
- "winnow",
-]
-
-[[package]]
-name = "toml_writer"
-version = "1.1.2+spec-1.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7d56353a2a665ad0f41a421187180aab746c8c325620617ad883a99a1cbe66d2"
-
 [[package]]
 name = "unicode-ident"
 version = "1.0.24"
@@ -429,25 +374,6 @@ version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "06abde3611657adf66d383f00b093d7faecc7fa57071cce2578660c9f1010821"
 
-[[package]]
-name = "walkdir"
-version = "2.5.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "29790946404f91d9c5d06f9874efddea1dc06c5efe94541a7d6863108e3a5e4b"
-dependencies = [
- "same-file",
- "winapi-util",
-]
-
-[[package]]
-name = "winapi-util"
-version = "0.1.11"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c2a7b1c03c876122aa43f3020e6c3c3ee5c05081c9a00739faf7503aeba10d22"
-dependencies = [
- "windows-sys",
-]
-
 [[package]]
 name = "windows-link"
 version = "0.2.1"
@@ -463,14 +389,14 @@ dependencies = [
  "windows-link",
 ]
 
-[[package]]
-name = "winnow"
-version = "1.0.4"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "23b97319f7b8343df12cc98938e5c3eb436064524c8d2b4e30a1d3a36eecdf81"
-
 [[package]]
 name = "xdg"
 version = "3.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2fb433233f2df9344722454bc7e96465c9d03bff9d77c248f9e7523fe79585b5"
+
+[[package]]
+name = "zmij"
+version = "1.0.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "29666d0abbfad1e3dc4dcf6144730dd3a3ab225bbbdac83319345b1b44ccfc1b"