fix: tolerate files vanishing mid-scan in find_download

The sort key in find_download() called item.stat() unprotected. The
search roots include the live Downloads folder, where Chrome renames
"*.crdownload" files to their final name between directory listing and
stat(), crashing the whole export with FileNotFoundError.

Stat once while collecting (skipping entries that raise OSError), then
sort the snapshot. This also halves the stat() calls per polling round.

Fixes the remaining race reported in #4.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skills/open-kimi-ppt/scripts/export_pptx.py

@@ -461,18 +461,23 @@ def find_download(
     last_sizes: Dict[Path, int] = {}
     stable: Dict[Path, int] = {}
     while time.monotonic() < deadline:
-        candidates: List[Path] = []
+        # Snapshot stats while collecting and tolerate races everywhere: the
+        # search roots include the live Downloads folder, where Chrome renames
+        # .crdownload files away between directory listing and stat().
+        entries: List[Tuple[Path, float, int]] = []
         for root in search_roots:
             if not root.exists():
                 continue
-            candidates.extend(path for path in root.rglob("*") if path.is_file())
-        for path in sorted(candidates, key=lambda item: item.stat().st_mtime, reverse=True):
-            try:
-                stat = path.stat()
-                size = stat.st_size
-                if since is not None and stat.st_mtime < since:
+            for path in root.rglob("*"):
+                if not path.is_file():
                     continue
-            except OSError:
+                try:
+                    info = path.stat()
+                except OSError:
+                    continue
+                entries.append((path, info.st_mtime, info.st_size))
+        for path, mtime, size in sorted(entries, key=lambda entry: entry[1], reverse=True):
+            if since is not None and mtime < since:
                 continue
             if size == last_sizes.get(path) and size > 0:
                 stable[path] = stable.get(path, 0) + 1