Walk an operand nested on another filesystem with -x (closes #106)

With -x, an operand under another operand is kept and walked on its
own when it is a directory on a different filesystem than the deepest
kept operand above it, or a regular file in such a directory, since
that operand's walk does not reach it. Below a kept operand, the rule
keeping records under a directory -x did not enter covers only
directories its own walk did not enter, so records of files gone from
it are deleted.

The new tests stub the devices and share their scan steps with the
test for #99, whose comment no
longer claims it runs every phase. The startWalk comment is rewrapped.

Model: opus-5-5

TODO.md

@@ -29,6 +29,12 @@
 
 # Completed Steps
 
+- with `-x`, an operand under another operand is walked on its own instead of
+  being dropped when it is a directory on a different filesystem than that
+  operand, or a regular file in such a directory, and the records of files gone
+  from below it are deleted (2026-10-14,
+  https://git.eeqj.de/sneak/sfdupes/issues/106)
+
 - 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)