refactor: restructure domain crate by bounded context

crates/domain/src/common/value_objects/checksum.rs

@@ -0,0 +1,29 @@
+use crate::common::errors::DomainError;
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash, serde::Serialize, serde::Deserialize)]
+pub struct Checksum(String);
+
+impl Checksum {
+    pub fn new(hex: impl Into<String>) -> Result<Self, DomainError> {
+        let hex = hex.into().to_lowercase();
+        if hex.len() != 64 {
+            return Err(DomainError::Validation(
+                format!("Checksum must be 64 hex characters, got {}", hex.len()),
+            ));
+        }
+        if !hex.chars().all(|c| c.is_ascii_hexdigit()) {
+            return Err(DomainError::Validation(
+                "Checksum contains non-hex characters".to_string(),
+            ));
+        }
+        Ok(Self(hex))
+    }
+
+    pub fn as_str(&self) -> &str { &self.0 }
+}
+
+impl std::fmt::Display for Checksum {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}