Refactor whitespace handling in JSON serialization and improve ISO-8601 timestamp formatting

- Removed unnecessary spaces in JSON key-value and array formatting for Python-style compatibility.
- Enhanced `format_iso8601_basic` to ensure precise microsecond-level time formatting.
- Updated `write_update_log` to return and store SHA-1 signatures for update log verification.

.replit

@@ -1,2 +0,0 @@
-language = "rust"
-run = "cargo test"

libips/src/repository/catalog.rs

@@ -66,7 +66,8 @@ pub type Result<T> = std::result::Result<T, CatalogError>;
 /// Format a SystemTime as an ISO-8601 'basic format' date in UTC
 pub fn format_iso8601_basic(time: &SystemTime) -> String {
     let datetime = convert_system_time_to_datetime(time);
-    format!("{}Z", datetime.format("%Y%m%dT%H%M%S.%f"))
+    let micros = datetime.timestamp_subsec_micros();
+    format!("{}.{:06}Z", datetime.format("%Y%m%dT%H%M%S"), micros)
 }
 
 /// Convert SystemTime to UTC DateTime, handling errors gracefully

libips/src/repository/file_backend.rs

@@ -2520,7 +2520,7 @@ impl FileBackend {
                 );
             }
 
-            let _ = catalog_writer::write_update_log(&update_log_path, &mut update_log)?;
+            let update_log_sig = catalog_writer::write_update_log(&update_log_path, &mut update_log)?;
             debug!("Wrote update log file");
 
             // Add an update log to catalog.attrs
@@ -2529,7 +2529,7 @@ impl FileBackend {
                 update_log_name.clone(),
                 crate::repository::catalog::UpdateLogInfo {
                     last_modified: timestamp.clone(),
-                    signature_sha1: None,
+                    signature_sha1: Some(update_log_sig),
                 },
             );