fix: read exportable through the exporter instance

scripts/exporter/base_exporter.py

@@ -81,9 +81,8 @@ class BaseExporter(ABC):
 
     # Shared helpers
 
-    @classmethod
     def exportable(
-        cls,
+        self,
         systems: dict[str, NativeSystem],
         require: str = "",
     ) -> list[tuple[NativeSystem, list[NativeFile]]]:
@@ -92,11 +91,12 @@ class BaseExporter(ABC):
         `require` names a hash the format cannot omit. An entry the format
         has no way to express is dropped here and counted by the caller,
         never written as a half entry the platform would read as a file that
-        can never verify.
+        can never verify. Read through the instance: RomM decides on the
+        keys its render has seen.
         """
         result: list[tuple[NativeSystem, list[NativeFile]]] = []
         for system in systems.values():
-            files = [fe for fe in system.files if fe.name and cls.writable(fe, require)]
+            files = [fe for fe in system.files if fe.name and self.writable(fe, require)]
             if files:
                 result.append((system, files))
         return result

scripts/exporter/romm_exporter.py

@@ -52,6 +52,12 @@ class Exporter(BaseExporter):
     def native_sources() -> dict[str, str]:
         return {"known_bios_files.json": SOURCE_URL}
 
+    def __init__(self) -> None:
+        super().__init__()
+        # Filled by render; empty until then, so writable() answers before it.
+        self._platform_keys: set[tuple[str, str]] = set()
+        self._first_addition: dict[tuple[str, str], int] = {}
+
     @staticmethod
     def _verifiable(fe: NativeFile) -> bool:
         return bool(fe.size()) and any(