refactor: simplify _dedup_central_directory using NameToInfo

NameToInfo is already a {filename -> latest ZipInfo} mapping because
each writestr/write call does NameToInfo[name] = zinfo, so later writes
overwrite earlier ones (confirmed via CPython zipfile source).

Replace the manual seen/deduped loop with a two-liner that uses public API:
- namelist() to get all filenames (preserving insertion order)
- dict.fromkeys() to deduplicate while keeping first-seen order
- getinfo() to retrieve the latest ZipInfo for each unique name

Only filelist needs to be updated; NameToInfo is already correct and
does not need to be rewritten. The new list is computed before assignment
so the ZipFile is never left in an inconsistent state on exception.

Made-with: Cursor

Author: Leo Ji

src/zarr/storage/_zip.py

@@ -136,24 +136,22 @@ def _dedup_central_directory(self) -> None:
         *last* (most recent) entry for every filename so that the on-disk central
         directory is clean.
 
-        Both ``filelist`` and ``NameToInfo`` are computed upfront and then
-        swapped in together so that the ZipFile object is never left in an
-        inconsistent state if an exception occurs mid-update.
+        ``NameToInfo`` is already a ``{filename: latest_ZipInfo}`` mapping because
+        each ``writestr``/``write`` call does ``NameToInfo[name] = zinfo``, so
+        later writes overwrite earlier ones.  We therefore only need to rebuild
+        ``filelist`` from the values of ``NameToInfo``; ``NameToInfo`` itself
+        requires no update.
+
+        The new list is computed before being assigned so that the ZipFile object
+        is never left in an inconsistent state if an exception is raised.
         """
         if self._zf.mode not in ("w", "a", "x"):
             return
-        seen: set[str] = set()
-        deduped: list[zipfile.ZipInfo] = []
-        for info in reversed(self._zf.filelist):
-            if info.filename not in seen:
-                seen.add(info.filename)
-                deduped.append(info)
-        new_filelist = list(reversed(deduped))
-        new_name_to_info = {info.filename: info for info in new_filelist}
-        # Swap both attributes together; if anything above raised, the
-        # original filelist/NameToInfo are still intact.
-        self._zf.filelist = new_filelist
-        self._zf.NameToInfo = new_name_to_info
+        # namelist() and getinfo() are public API.
+        # dict.fromkeys preserves first-seen order while deduplicating names;
+        # getinfo() returns the latest ZipInfo for each name (NameToInfo[name]).
+        unique_names = dict.fromkeys(self._zf.namelist())
+        self._zf.filelist = [self._zf.getinfo(name) for name in unique_names]
 
     async def clear(self) -> None:
         # docstring inherited