Kitty has an arbitrary file overwrite via symlink following in `kitten dnd` remote drop staging

Summary

Kitty is a cross-platform GPU based terminal. In versions 0.47.0 and 0.47.1, `kitten dnd` can allow a malicious remote drag-and-drop source to overwrite or truncate arbitrary files writable by the local kitty user. Remote `text/uri-list` drops are staged in a temporary directory, but on case-sensitive filesystems duplicate remote basenames are not de-duplicated. An attacker can first create a staged symlink and then send a same-name regular-file entry. The regular-file write uses `utils.CreateAt()` / `openat(O_RDWR|O_CREAT|O_TRUNC)` without `O_NOFOLLOW`, so it follows the attacker-created symlink and writes outside the staging directory before final overwrite confirmation runs. This appears related in class to the file-transfer symlink advisory, but it is a different bug: it affects `kitten dnd` remote drag-and-drop staging, uses different vulnerable code (`kittens/dnd/drop.go` and `tools/utils/file_at_fd.go`), and reproduces on commit `4aa4a5c0567a92553a8c20a88a4352da637fca5d`, after the file-transfer `O_NOFOLLOW` fix. Version 0.47.2 patches the issue.