fix: additions join the native id their cores read

A platform system spanning several native ids with no native_id filed every addition under the id that sorts first: RetroDECK's arcade wrote MAME device archives as FBNeo BIOS, RetroBat's Amiga put A500 kickstarts under amiga1200. An addition now joins the id whose declared files its cores read, and one the cores do not settle is counted and not written.

scripts/export_native.py

@@ -776,6 +776,8 @@ def export_platform(
             # A content-scanning core's names are labels: a list checked
             # by path cannot hold them.
             (report.labels, "entry", "entries", "found by content"),
+            (len(report.unattributed), "file", "",
+             "no single native system of the platform takes"),
         )
         if number
     ]
@@ -833,6 +835,7 @@ def export_platform(
         "requirements_corrected": requirements,
         "withheld": list(withheld),
         "found_by_content": report.labels,
+        "unattributed": report.unattributed,
         "removal_candidates": candidates,
         "doubled": doubled,
         "invalid": issues,