fix: write each export as its bytes and mode

write_text translated line ends on Windows, so FirmwareDatabase.cs's
CRLF would have come out CR CR LF; it replaced the published file in
place, so a run stopped mid-write left it truncated; and it dropped the
mode, while rocknix-systems is 100755 in ROCKNIX's tree. Files are
written atomically as UTF-8 bytes, and an exporter states the mode of
any file the platform does not publish as 0644.

scripts/exporter/base_exporter.py

@@ -126,6 +126,11 @@ class BaseExporter(ABC):
         """
         return True
 
+    @staticmethod
+    def file_modes() -> dict[str, int]:
+        """The mode of each file the platform publishes other than 0644."""
+        return {}
+
     @staticmethod
     def keys_by_system() -> bool:
         """Whether the platform finds a file through its system's own name.

scripts/exporter/rocknix_exporter.py

@@ -77,6 +77,11 @@ class Exporter(BatoceraExporter):
     def native_filename() -> str:
         return "rocknix-systems"
 
+    @staticmethod
+    def file_modes() -> dict[str, int]:
+        # 100755 in ROCKNIX's tree at the pinned tag (git/trees 20261001).
+        return {"rocknix-systems": 0o755}
+
     @staticmethod
     def native_sources() -> dict[str, str]:
         return {"rocknix-systems": SOURCE_URL, EMULATORS_MK: EMULATORS_URL}