feat(core): add backup deps + error variants

Adds zstd, tar, base64 to relicario-core; introduces
BackupBadMagic / BackupUnsupportedVersion / BackupSchemaMismatch.
Foundation for the backup module landing in Task 2.

crates/relicario-core/Cargo.toml

@@ -26,5 +26,8 @@ chrono = { version = "0.4", default-features = false, features = ["serde", "cloc
 hex = "0.4"
 url = { version = "2", features = ["serde"] }
 getrandom = "0.2"
+zstd = { version = "0.13", default-features = false }
+tar = { version = "0.4", default-features = false }
+base64 = "0.22"
 
 [dev-dependencies]

crates/relicario-core/src/error.rs

@@ -39,6 +39,18 @@ pub enum RelicarioError {
     #[error("unsupported vault format version: found 0x{found:02x}, expected 0x{expected:02x}")]
     UnsupportedFormatVersion { found: u8, expected: u8 },
 
+    /// Backup file's first 4 bytes don't match the "RBAK" magic.
+    #[error("not a relicario backup file")]
+    BackupBadMagic,
+
+    /// Backup format version is newer than this binary supports.
+    #[error("backup created by a newer relicario; upgrade required")]
+    BackupUnsupportedVersion { found: u8, expected: u8 },
+
+    /// Backup envelope schema version doesn't match.
+    #[error("backup envelope schema v{found}; this relicario reads v{expected}")]
+    BackupSchemaMismatch { found: u32, expected: u32 },
+
     /// An item was looked up by ID but does not exist in the manifest.
     #[error("item not found: {0}")]
     ItemNotFound(String),
@@ -130,4 +142,18 @@ mod tests {
         assert!(s.contains("01") || s.contains("1"));
         assert!(s.contains("02") || s.contains("2"));
     }
+
+    #[test]
+    fn backup_errors_carry_useful_messages() {
+        let bad = RelicarioError::BackupBadMagic;
+        assert!(format!("{}", bad).contains("not a relicario backup file"));
+
+        let ver = RelicarioError::BackupUnsupportedVersion { found: 0x02, expected: 0x01 };
+        let s = format!("{}", ver);
+        assert!(s.contains("newer"));
+
+        let schema = RelicarioError::BackupSchemaMismatch { found: 2, expected: 1 };
+        let s = format!("{}", schema);
+        assert!(s.contains("v2") && s.contains("v1"));
+    }
 }