fix: state only what lands in the export summary

The summary in exports/<platform>.json read '82 hashes corrected' and '854 additions left out'. It now counts what the written file carries, in the exports page's terms (hashes updated to the emulator source, required flags aligned with the code). Files the format cannot carry, withheld updates and content-found labels are printed on a line of their own and stay in the report's counts.

tests/test_export_report.py

@@ -26,7 +26,7 @@ def _report(platform: str, files: list[str], **changes: object) -> dict:
         "platform": platform,
         "originals": {f: f"https://example.org/{platform}/{f}" for f in files},
         "files": files,
-        "summary": "3 kept, 1 added, 7 hashes corrected, 0 requirements corrected",
+        "summary": "3 kept, 1 file added, 7 hashes updated to the emulator source",
         "hashes_corrected": [], "hashes_filled": [], "sizes_corrected": [],
         "sizes_removed": [], "hashes_cleared": [], "requirements_corrected": 0,
         "withheld": [], "invalid": [],
@@ -157,6 +157,75 @@ class ThePageAddressesMaintainersAsPeers(unittest.TestCase):
             self.assertNotIn(word, page)
 
 
+class TheRunReportStatesWhatTheFileCarries(unittest.TestCase):
+    """exports/<platform>.json, the report beside the file offered to a
+    maintainer, said "82 hashes corrected" and "854 additions left out":
+    the words the exports page avoids, and counts of things not in the file
+    mixed into the summary of what is."""
+
+    JUDGING = ("their", "corrected", "wrong", "drift", "left out", "never seen")
+
+    def test_summary_and_messages(self):
+        import json  # noqa: PLC0415
+        from unittest import mock  # noqa: PLC0415
+
+        from exporter.base_exporter import BaseExporter  # noqa: PLC0415
+
+        class Plain(BaseExporter):
+            @staticmethod
+            def platform_name() -> str:
+                return "plat"
+
+            @staticmethod
+            def native_filename() -> str:
+                return "list.json"
+
+            @staticmethod
+            def requires() -> str:
+                return "md5"
+
+            def render(self, systems, report, originals, scraped=None):
+                return {"list.json": json.dumps(sorted(
+                    fe.name for system in systems.values()
+                    for fe in system.files if self.writable(fe)
+                ))}
+
+            def validate(self, systems, produced):
+                return []
+
+        truth = {"systems": {"psx": {"files": [
+            {"name": "a.bin", "md5": "b" * 32, "validation": ["md5"]},
+            {"name": "new.bin", "md5": "c" * 32},
+            {"name": "nohash.bin", "size": 16},
+        ]}}}
+        scraped = {"systems": {"psx": {"native_id": "psx", "files": [
+            {"name": "a.bin", "destination": "a.bin", "native_system": "psx",
+             "md5": "a" * 32},
+        ]}}}
+        with tempfile.TemporaryDirectory(dir=ROOT / "tmp") as scratch, \
+                mock.patch.object(export_native, "load_inputs",
+                                  return_value=(truth, scraped)), \
+                mock.patch.object(export_native, "unpinned", return_value=[]), \
+                mock.patch.object(export_native, "removal_candidates", return_value=[]):
+            ok, messages = export_native.export_platform(
+                "plat", Plain, Path(scratch), Path(scratch) / "out",
+                str(ROOT / "platforms"), Path(scratch) / "cache", False,
+            )
+            self.assertTrue(ok, messages)
+            report = json.loads((Path(scratch) / "out" / "plat.json").read_text())
+        self.assertEqual(
+            report["summary"],
+            "1 kept, 1 file added, 1 hash updated to the emulator source",
+        )
+        self.assertIn(
+            "not in the file: 1 file the format or its checker cannot carry", messages
+        )
+        self.assertIn("hash updated: psx/a.bin md5", messages)
+        for text in [report["summary"], *messages]:
+            for word in self.JUDGING:
+                self.assertNotIn(word, text.lower())
+
+
 UPSTREAM_CACHE = ROOT / ".cache" / "upstream-native"
 _DAT_ROM = re.compile(r"^\trom \( name (\"[^\"]+\"|\S+) (.*) \)$")
 _DAT_FIELDS = {"size": "size", "crc": "crc32", "md5": "md5", "sha1": "sha1"}