fix(core,cli): harden backup-restore tar unpack against path traversal (audit S2)

cmd_backup_restore previously called tar::Archive::unpack with default
settings, allowing malicious .relbak archives to escape the target
directory via .. entries, absolute paths, or symlinks. No size cap
meant tar bombs could exhaust disk space.

Replaced with relicario_core::safe_unpack_git_archive which:
- Rejects .. (ParentDir), absolute (RootDir), and drive-prefix
  (Prefix) components with "path traversal blocked" error.
- Rejects symlinks and hardlinks outright.
- Checks declared header size before reading body; rejects entries or
  cumulative totals exceeding the caller's cap.
- Returns (relative-path, bytes) pairs; the CLI re-checks
  dest.starts_with(git_dir) after OS-level path resolution.
- CLI cap: min(100 × compressed size, 1 GiB).

Acceptance: 5 unit tests in relicario-core (traversal, absolute path,
symlink, size bomb, happy path); existing CLI backup roundtrip tests
remain green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/relicario-core/tests/safe_unpack.rs

@@ -0,0 +1,187 @@
+use std::path::PathBuf;
+use tar::{Builder, Header, EntryType};
+use relicario_core::safe_unpack_git_archive;
+
+/// Craft a raw POSIX ustar tar with a single entry using the given raw path bytes.
+/// The tar crate's `Builder` sanitises paths, so we write the 512-byte header
+/// manually to produce truly malicious archives.
+fn raw_tar_with_path(raw_path: &[u8], content: &[u8]) -> Vec<u8> {
+    let mut buf = vec![0u8; 512]; // one header block
+
+    // Bytes 0-99: name field (null-padded)
+    let name_len = raw_path.len().min(100);
+    buf[..name_len].copy_from_slice(&raw_path[..name_len]);
+
+    // Bytes 100-107: mode = "0000644\0"
+    buf[100..108].copy_from_slice(b"0000644\0");
+
+    // Bytes 108-115: uid
+    buf[108..116].copy_from_slice(b"0000000\0");
+
+    // Bytes 116-123: gid
+    buf[116..124].copy_from_slice(b"0000000\0");
+
+    // Bytes 124-135: size (octal, 11 digits + null)
+    let size_str = format!("{:011o}\0", content.len());
+    buf[124..136].copy_from_slice(size_str.as_bytes());
+
+    // Bytes 136-147: mtime
+    buf[136..148].copy_from_slice(b"00000000000\0");
+
+    // Bytes 148-155: checksum placeholder (spaces during compute)
+    buf[148..156].copy_from_slice(b"        ");
+
+    // Byte 156: typeflag = '0' (regular file)
+    buf[156] = b'0';
+
+    // Bytes 257-262: magic "ustar\0"
+    buf[257..263].copy_from_slice(b"ustar\0");
+    // Bytes 263-264: version "00"
+    buf[263..265].copy_from_slice(b"00");
+
+    // Compute checksum (sum of all bytes, checksum field treated as spaces).
+    let checksum: u32 = buf.iter().map(|&b| b as u32).sum();
+    let cksum_str = format!("{:06o}\0 ", checksum);
+    buf[148..156].copy_from_slice(cksum_str.as_bytes());
+
+    // Append padded content blocks.
+    let mut out = buf;
+    if !content.is_empty() {
+        out.extend_from_slice(content);
+        // Pad to 512-byte boundary.
+        let remainder = content.len() % 512;
+        if remainder != 0 {
+            out.extend(std::iter::repeat(0u8).take(512 - remainder));
+        }
+    }
+
+    // Two zero blocks = end-of-archive.
+    out.extend(std::iter::repeat(0u8).take(1024));
+    out
+}
+
+/// Build a tar with a raw symlink entry (typeflag = '2').
+fn raw_symlink_tar() -> Vec<u8> {
+    let mut buf = vec![0u8; 512];
+
+    // name
+    buf[..9].copy_from_slice(b"evil_link");
+    // mode
+    buf[100..108].copy_from_slice(b"0000755\0");
+    // uid/gid
+    buf[108..116].copy_from_slice(b"0000000\0");
+    buf[116..124].copy_from_slice(b"0000000\0");
+    // size = 0
+    buf[124..136].copy_from_slice(b"00000000000\0");
+    // mtime
+    buf[136..148].copy_from_slice(b"00000000000\0");
+    // checksum placeholder
+    buf[148..156].copy_from_slice(b"        ");
+    // typeflag = '2' (symlink)
+    buf[156] = b'2';
+    // linkname
+    let target = b"/etc/passwd";
+    buf[157..157 + target.len()].copy_from_slice(target);
+    // magic
+    buf[257..263].copy_from_slice(b"ustar\0");
+    buf[263..265].copy_from_slice(b"00");
+
+    // Compute checksum.
+    let checksum: u32 = buf.iter().map(|&b| b as u32).sum();
+    let cksum_str = format!("{:06o}\0 ", checksum);
+    buf[148..156].copy_from_slice(cksum_str.as_bytes());
+
+    let mut out = buf;
+    out.extend(std::iter::repeat(0u8).take(1024)); // end-of-archive
+    out
+}
+
+fn build_normal_tar() -> Vec<u8> {
+    let mut buf = Vec::new();
+    {
+        let mut builder = Builder::new(&mut buf);
+        let content = b"hello";
+        let mut header = Header::new_gnu();
+        header.set_entry_type(EntryType::Regular);
+        header.set_size(content.len() as u64);
+        header.set_cksum();
+        builder
+            .append_data(&mut header, "subdir/hello.txt", content.as_ref())
+            .unwrap();
+        builder.finish().unwrap();
+    }
+    buf
+}
+
+fn build_oversize_tar() -> Vec<u8> {
+    // Actual 2048-byte body; test will use cap=1024
+    let mut buf = Vec::new();
+    {
+        let mut builder = Builder::new(&mut buf);
+        let content = vec![0u8; 2048];
+        let mut header = Header::new_gnu();
+        header.set_entry_type(EntryType::Regular);
+        header.set_size(content.len() as u64);
+        header.set_cksum();
+        builder
+            .append_data(&mut header, "bigfile.bin", content.as_slice())
+            .unwrap();
+        builder.finish().unwrap();
+    }
+    buf
+}
+
+#[test]
+fn restore_rejects_path_traversal() {
+    // Craft a tar with "../../escaped.txt" using raw bytes (Builder sanitises paths).
+    let bytes = raw_tar_with_path(b"../../escaped.txt", b"evil content");
+    let err = safe_unpack_git_archive(&bytes, 1024 * 1024).unwrap_err();
+    let msg = format!("{err:#}");
+    assert!(
+        msg.contains("path traversal") || msg.contains(".."),
+        "got: {msg}"
+    );
+}
+
+#[test]
+fn restore_rejects_absolute_path() {
+    // Craft a tar with "/etc/escaped.txt" using raw bytes.
+    let bytes = raw_tar_with_path(b"/etc/escaped.txt", b"evil content");
+    let err = safe_unpack_git_archive(&bytes, 1024 * 1024).unwrap_err();
+    let msg = format!("{err:#}");
+    assert!(
+        msg.contains("path traversal") || msg.contains("absolute"),
+        "got: {msg}"
+    );
+}
+
+#[test]
+fn restore_rejects_symlink() {
+    let bytes = raw_symlink_tar();
+    let err = safe_unpack_git_archive(&bytes, 1024 * 1024).unwrap_err();
+    let msg = format!("{err:#}");
+    assert!(
+        msg.contains("symlink") || msg.contains("link"),
+        "got: {msg}"
+    );
+}
+
+#[test]
+fn restore_rejects_size_bomb() {
+    let bytes = build_oversize_tar(); // actual 2048-byte entry
+    let err = safe_unpack_git_archive(&bytes, 1024).unwrap_err(); // cap = 1024 bytes
+    let msg = format!("{err:#}");
+    assert!(
+        msg.contains("size") || msg.contains("cap") || msg.contains("too large"),
+        "got: {msg}"
+    );
+}
+
+#[test]
+fn restore_accepts_normal_files() {
+    let buf = build_normal_tar();
+    let entries = safe_unpack_git_archive(&buf, 1024 * 1024).expect("happy path");
+    assert_eq!(entries.len(), 1);
+    assert_eq!(entries[0].0, PathBuf::from("subdir/hello.txt"));
+    assert_eq!(entries[0].1, b"hello");
+}