Never scan an operand inside a .zfs directory (closes #142)

An operand inside a directory named .zfs was walked, because the
operand check looked only at the operand's own name. The check now
also looks at every directory above the operand, both as given and
with its symlinks resolved. Such an operand gets a warning, counts as
skipped and is not walked, whether given alone or under another
operand. The check runs before overlapping operands are pruned, so an
operand under another one still gets its warning.

The test for an operand named .zfs now writes its record beneath .zfs
straight into the database, since no scan stores one there any more.

Judgement call: a path component named .zfs counts whatever it is, a
symlink included.

Model: opus-5-5

TODO.md

@@ -29,6 +29,11 @@
 
 # Completed Steps
 
+- an operand at or inside a directory named `.zfs` is never scanned, whether
+  given alone or under another operand. `scan` warns about it, counts it as
+  skipped and does not walk it (2026-10-26,
+  https://git.eeqj.de/sneak/sfdupes/issues/142)
+
 - a file or directory removed while the scan runs is not an error, in the walk
   or in the hash phase. The scan prints no line for it and does not count it as
   skipped. When the first of several hard links to a file is gone, the hash