Escape tab, newline, CR and backslash in report paths (closes #7)

A path holding a tab or newline split a row of the report or trees
output. The path columns of both now write a backslash, tab, newline
and carriage return as \\, \t, \n and \r; every other byte is written
unchanged. Grouping and sorting still use the stored path. Warnings
on stderr are escaped the same way in warnf, so each stays one line.

In trees, the root directory's node now has the path "/" instead of
an empty string, and its children's paths start with a single slash.

README states the rule under "Report output format".

Model: opus-5-5

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- escape tabs, newlines, carriage returns and backslashes in report,
+  trees and warning paths; the root directory's path is `/`
+  (2026-10-03, https://git.eeqj.de/sneak/sfdupes/issues/7)
+
 - stamp the git tag or short commit in a plain `docker build .`
   instead of `dev` (2026-10-02, branch `next`, closes
   https://git.eeqj.de/sneak/sfdupes/issues/67): `.dockerignore` now