Introduce target-aware resolved reference storage

docs/integrations/json-output.md

@@ -100,8 +100,9 @@ Fields with empty values may be omitted by JSON encoding.
 
 `source_digests` contains source document digests only. Bound references are
 recorded separately under `references`, which contains provenance only:
-lane ID, slot name, origin type and URI, digest, media type, byte size, and
-binding source. Reference content is not written to durable output.
+target stage, lane ID when present, slot name, origin type and URI, digest,
+media type, byte size, and binding source. Reference content is not written to
+durable output.
 
 When references are bound, the manifest section has this shape:
 
@@ -109,6 +110,7 @@ When references are bound, the manifest section has this shape:
 {
   "references": [
     {
+      "stage": "extract",
       "lane_id": "spells",
       "slot_name": "roster",
       "origin_type": "file",