Feature: add export package with StreamingManifestWriter and _dumps

src/documents/export/sinks.py

@@ -0,0 +1,43 @@
+from __future__ import annotations
+
+import json
+from typing import TYPE_CHECKING
+
+from django.core.serializers.json import DjangoJSONEncoder
+
+if TYPE_CHECKING:
+    from typing import TextIO
+
+
+def _dumps(content: list | dict) -> str:
+    """Serialize export JSON consistently across all sinks."""
+    return json.dumps(content, cls=DjangoJSONEncoder, indent=2, ensure_ascii=False)
+
+
+class StreamingManifestWriter:
+    """Incrementally writes a JSON array to a text handle, one record at a time.
+
+    Knows nothing about folders or zips: it writes the array framing and records
+    to whatever handle the sink's ``stream()`` yields. The sink owns the handle's
+    lifecycle (atomic rename, compare, spooling).
+    """
+
+    def __init__(self, handle: TextIO) -> None:
+        self._file = handle
+        self._first = True
+        self._file.write("[")
+
+    def write_record(self, record: dict) -> None:
+        if not self._first:
+            self._file.write(",\n")
+        else:
+            self._first = False
+        self._file.write(_dumps(record))
+
+    def write_batch(self, records: list[dict]) -> None:
+        for record in records:
+            self.write_record(record)
+
+    def close(self) -> None:
+        """Write the closing bracket. Does NOT close the handle (the sink owns it)."""
+        self._file.write("\n]")