fix: fix did derivation and signature generation.

This fixes two bugs that were affecting did derivation and signature
generation.

- The `UnsignedOperation` and `Operation` types were not serializing
prev as `null` and were instead skipping serialization when it was
`None`.
- The DID was being calculated from the hash of the JSON encoding
instead of the CBOR encoding.

src/builder.rs

···
 use crate::crypto::SigningKey;
 use crate::did::Did;
 use crate::document::ServiceEndpoint;
-use crate::encoding::{base32_encode, sha256};
+use crate::encoding::{base32_encode, dag_cbor_encode, sha256};
 use crate::error::{PlcError, Result};
 use crate::operations::{Operation, UnsignedOperation};
 use crate::validation::{
···
 
         // The DID is derived from the CID by taking the hash portion
         // For simplicity, we'll hash the entire serialized operation
-        let serialized = serde_json::to_vec(operation)
-            .map_err(|e| PlcError::DagCborError(e.to_string()))?;
+        let serialized =
+            dag_cbor_encode(operation).map_err(|e| PlcError::DagCborError(e.to_string()))?;
 
         let hash = sha256(&serialized);
         let encoded = base32_encode(&hash);

src/operations.rs

···
         services: HashMap<String, ServiceEndpoint>,
 
         /// Previous operation CID (null for genesis)
-        #[serde(skip_serializing_if = "Option::is_none")]
         prev: Option<String>,
 
         /// Base64url-encoded signature
···
         service: String,
 
         /// Previous operation CID
-        #[serde(skip_serializing_if = "Option::is_none")]
         prev: Option<String>,
 
         /// Base64url-encoded signature
···
         services: HashMap<String, ServiceEndpoint>,
 
         /// CID of previous operation (None for genesis)
-        #[serde(skip_serializing_if = "Option::is_none")]
         prev: Option<String>,
     },
 
···
         service: String,
 
         /// CID of previous operation (None for genesis)
-        #[serde(skip_serializing_if = "Option::is_none")]
         prev: Option<String>,
     },
 }