Keep records on other filesystems under an operand with -x (closes #99)

With -x the walk does not enter a directory on another filesystem than
its operand, but the update phase still deleted every record below it.
A separate scan of a filesystem mounted under the operand therefore lost
its records on every -x scan and re-hashed its whole tree.

The walk now sends an event naming each directory it declines this way,
and the walk phase treats the records below those directories like
records outside the operands: they are not deleted, and their sizes join
the size census. README "scan mode" and "Rules for the walk" state the
rule.

Model: opus-5-5

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- with `-x`, `scan` keeps the records below a directory on another filesystem
+  under its operand, and counts their sizes in the size census, instead of
+  deleting them (2026-10-14, https://git.eeqj.de/sneak/sfdupes/issues/99)
+
 - on a terminal, a warning for the last file of a phase no longer erases the
   phase's finished bar: the bar is drawn again below the warning (2026-10-14,
   https://git.eeqj.de/sneak/sfdupes/issues/102)