Document accepted chunk map export

docs/integrations/json-output.md

@@ -18,6 +18,8 @@ The encoder writes:
 - `lanes/<lane-id>.json`, one file per normalized serialized artifact
 - `rejected.json`
 - `warnings.json`
+- `chunk-map.json`, only when the JSON output binding enables
+  `include_chunk_map` and the run has an accepted chunk map
 
 Files are pretty-printed JSON with a trailing newline when the payload is JSON.
 Logical file paths are relative, slash-separated, and may not contain `..`.
@@ -60,6 +62,13 @@ contains the normalized payload `media_type`, normalizer `module_key`, and
 response `schema_id`, `schema_name`, and `schema_version` when those values are
 available.
 
+When present, the top-level optional `chunk_map` descriptor contains exactly
+`artifact_kind`, `file`, `media_type`, `schema_id`, `schema_name`, and
+`schema_version`. It identifies the pipeline-wide `chunk-map.json`; it is not
+a lane output and never appears in `output_files`. The descriptor and file are
+both absent when export is disabled or no chunk plan was accepted. Its payload
+contract is defined by [Accepted Chunk Map](chunk-map.md).
+
 ## `manifest.json`
 
 `manifest.json` contains a run manifest. This abridged example shows its core