feat(client): add merge_bom_json method for assembly BOM merge

Forward-looking JSON merge endpoint for POST /api/items/{pn}/bom/merge.
Not called by Phase 1 (which uses individual CRUD calls). Ready for
Phase 2 when the server endpoint ships.

Refs: #276

silo_client/__init__.py

@@ -803,6 +803,24 @@ class SiloClient:
         pn = urllib.parse.quote(part_number, safe="")
         return self._upload_ods(f"/items/{pn}/bom/merge", ods_bytes, filename)
 
+    def merge_bom_json(self, part_number: str, entries: list) -> Dict[str, Any]:
+        """Submit a JSON BOM merge from assembly extraction.
+
+        Calls ``POST /api/items/{partNumber}/bom/merge`` with a JSON body
+        containing assembly-derived BOM entries.  The server applies adds
+        and quantity changes, flags removed items as unreferenced, and
+        returns the diff.
+
+        Not used by Phase 1 (which calls add/update individually).
+        Ready for Phase 2 when the server endpoint ships.
+        """
+        pn = urllib.parse.quote(part_number, safe="")
+        return self._request(
+            "POST",
+            f"/items/{pn}/bom/merge",
+            {"source": "assembly", "entries": entries},
+        )
+
     # -- Schemas ------------------------------------------------------------
 
     def list_schemas(self) -> list: