fix: type pack helpers as ZipFile or PackRecord

Six helpers that write a pack member declared zipfile.ZipFile while every
call site also passes the PackRecord the pipeline builds emulator and
narrowed packs with. mypy reported the 19 call sites as arg-type errors.
With the union in the signature a helper that calls writestr or write
without its isinstance branch is a union-attr error in CI, instead of an
AttributeError in the pipeline's record pass.

scripts/generate_pack.py

@@ -164,7 +164,9 @@ class PackRecord:
         self.sources: dict[str, str] = {}
 
 
-def _add_pack_member(zf: zipfile.ZipFile, source_path: str, arcname: str) -> None:
+def _add_pack_member(
+    zf: zipfile.ZipFile | PackRecord, source_path: str, arcname: str
+) -> None:
     """Add a file to the pack with metadata that does not depend on the build.
 
     ZipFile.write copies the source file's mtime into the member. For a file
@@ -197,7 +199,9 @@ def _add_pack_member(zf: zipfile.ZipFile, source_path: str, arcname: str) -> Non
         shutil.copyfileobj(source, target, 1024 * 1024)
 
 
-def _write_generated_member(zf: zipfile.ZipFile, arcname: str, text: str) -> None:
+def _write_generated_member(
+    zf: zipfile.ZipFile | PackRecord, arcname: str, text: str
+) -> None:
     """Write a pack member the builder composes rather than copies.
 
     READMEs, instruction notes and the manifest are produced at build time,
@@ -581,7 +585,7 @@ def _data_directory_members(
 
 
 def _pack_data_directories(
-    zf: zipfile.ZipFile,
+    zf: zipfile.ZipFile | PackRecord,
     pack_systems: dict,
     data_registry: dict | None,
     platform_name: str,
@@ -1476,7 +1480,9 @@ def generate_pack(
 
 
 def _extract_zip_to_archive(
-    source_zip: str, dest_prefix: str, target_zf: zipfile.ZipFile
+    source_zip: str,
+    dest_prefix: str,
+    target_zf: zipfile.ZipFile | PackRecord,
 ) -> None:
     """Extract contents of a source ZIP into target ZIP under dest_prefix."""
     with zipfile.ZipFile(source_zip, "r") as src:
@@ -1507,7 +1513,7 @@ def _extract_zip_to_archive(
 def _add_zip_to_pack(
     source_zip: str,
     dest_path: str,
-    target_zf: zipfile.ZipFile,
+    target_zf: zipfile.ZipFile | PackRecord,
     file_entry: dict | None = None,
 ) -> None:
     """Write a source ZIP into the pack, byte for byte.
@@ -1555,7 +1561,7 @@ class DamagedArchiveError(OSError):
 
 
 def _pack_data_tree(
-    zf: zipfile.ZipFile,
+    zf: zipfile.ZipFile | PackRecord,
     local_cache: str,
     dd_dest: str,
     seen_destinations: set[str],