[lukisch]

Follow-up to #1, found in the second review. This PR targets the branch of #1 so that #1 picks up the fix.

Problem

edit_library_item deletes the old entry first, then calls save_snippet and ignores the result. #1 makes save_snippet reject names containing : or ending in .. Renaming a snippet to Regex: Gruppen or Kap. 1. therefore deletes it without any message. On master those renames succeed (the same loss already existed there for names containing /).

Change

• New LibraryManager.rename_snippet: saves under the new name first and removes the old entry only if that succeeded. If old and new name resolve to the same file (case-only rename on a case-insensitive file system), the delete is skipped.
• edit_library_item uses it and shows a warning when saving fails, also for a plain edit without rename.
• CHANGELOG entry.

Verification

• pytest -q (Linux, Python 3.11, PySide6 offscreen): 131 passed (126 before, 5 new).
• New tests cover: rejected names keep the old snippet, valid rename moves it, locked target keeps both, same-file rename keeps the file, and the dialog calls rename_snippet and reports failures.
• ruff check: 368 findings versus 369 on the base, none new. The repository does not lint clean today.

Not covered

• Not run on Windows locally; the case-only rename path is tested with a mocked os.path.samefile.
• The dialog path is checked by source inspection, not by driving the Qt dialog.
• The other notes from the review of fix(library,editor): block snippet path traversal, preserve line endings on save #1 (names ending in a dot rejected for snippets too, control characters accepted, changelog wording on mixed line endings) are unchanged.

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