Copy serialization description from existing manifest

Instead of creating a new serialization description from scratch,
copy all parameters (hash_type, allow_symlinks, ignore_paths) from
the existing manifest's serialization type. This ensures consistency
across incremental updates by preserving the original manifest's
serialization configuration.

Author: edonadei

src/model_signing/_serialization/incremental.py

@@ -95,12 +95,27 @@ def __init__(
                 if isinstance(item, manifest._File):
                     self._existing_items[item.path] = item
 
-        # Precompute serialization description
-        hasher = file_hasher_factory(pathlib.Path())
-        self._serialization_description = manifest._FileSerialization(
-            hasher.digest_name, self._allow_symlinks, self._ignore_paths
-        )
-        self._is_blake3 = hasher.digest_name == "blake3"
+        # Copy serialization description from existing manifest
+        # (similar to C++ copy constructor)
+        if isinstance(
+            existing_manifest._serialization_type, manifest._FileSerialization
+        ):
+            # Copy from file-based serialization
+            self._serialization_description = manifest._FileSerialization(
+                existing_manifest._serialization_type._hash_type,
+                existing_manifest._serialization_type._allow_symlinks,
+                existing_manifest._serialization_type._ignore_paths,
+            )
+            self._is_blake3 = (
+                existing_manifest._serialization_type._hash_type == "blake3"
+            )
+        else:
+            # Fall back to creating new one for shard-based manifests
+            hasher = file_hasher_factory(pathlib.Path())
+            self._serialization_description = manifest._FileSerialization(
+                hasher.digest_name, self._allow_symlinks, self._ignore_paths
+            )
+            self._is_blake3 = hasher.digest_name == "blake3"
 
     def set_allow_symlinks(self, allow_symlinks: bool) -> None:
         """Set whether following symlinks is allowed."""