fix: harden getStoragePath and keep CRLF endings

- Normalize the returned path to forward slashes so it matches what
  Storage.list() (glob) returns, instead of leaking `\` on Windows.
- Treat the storage root itself as "not inside", so passing a directory
  no longer yields an empty path that Storage.load() would open as a dir.
- Extract the containment check into isInsideDirectory() and drop the
  redundant isAbsolute(filepath) branch (path.resolve already handles it).
- Restore the repository's CRLF line endings in the nested fixture.

tests/__data__/input/playlist_format/nested/nested.m3u

@@ -1,5 +1,5 @@
-#EXTM3U
-#EXTINF:-1 tvg-id="",Zulu (720p)
-https://example.com/zulu.m3u8
-#EXTINF:-1 tvg-id="",Alpha (720p)
-https://example.com/alpha.m3u8
+#EXTM3U
+#EXTINF:-1 tvg-id="",Zulu (720p)
+https://example.com/zulu.m3u8
+#EXTINF:-1 tvg-id="",Alpha (720p)
+https://example.com/alpha.m3u8