feat: record each export change before and after

The page named the entries a change touched and never the values, so
a maintainer could not check a correction. The report now carries
changes, one object per change with kind, entry, field, old and new,
read from the platform entry and from the value the render writes
(written() per format: record value for System.dat and RomM, one md5
where a single one is checked). The existing label lists are derived
from it. The page writes "md5 X becomes Y" and names each required
flag with its new value. A test parses the original and updated
System.dat and es_bios.xml and checks the list equals the diff.

scripts/exporter/systemdat_exporter.py

@@ -154,6 +154,10 @@ class Exporter(BaseExporter):
         )
         return lines
 
+    def written(self, fe: NativeFile, field_name: str) -> str:
+        """A record holds one value per field, the one record_hash names."""
+        return fe.record_hash(field_name)
+
     def fills(self, fe: NativeFile, field_name: str) -> bool:
         return self.states(fe, field_name) and self.record_fills(fe, field_name)