FileHistory backup-copy fallback now uses atomic staged write with integrity check and retry

When Claude Code's file-history feature can't create a backup of a session's files by hard-linking (a fast way to duplicate a file without copying its data) and falls back to copying instead, it now writes to a temporary file first, checks that the copied file's size matches the original, and only then renames it into place. If the rename fails, it retries for about 1.5 seconds before giving up, logging a specific message if it's ultimately refused.