Cosmetic: normalize double-backslash \\n / \\t to single-backslash \n / \t
An older version of ShellCheck once recommended escaping backslash-n and
backslash-t as \\n / \\t in double-quoted strings; a later version reversed
that guidance since it was unnecessary. Both forms behave identically at
runtime, but the double-backslash form is visual clutter. Applied via
tools/sed_fixer.sh across the tree; no functional change.