scan: never record its own database files (closes #115)

When the database lay under a scanned operand, the walk recorded the
database file, its -wal and -shm files and its .lock file like any other
file, so every rescan updated them and report paired them with empty
files. Once the database and the lock file are open, the scan now notes
their device and inode, and the walk passes over a file with one of
them, as an operand or below one, printing no line and counting
nothing. Matching by device and inode holds through a symlinked
directory, where comparing paths would not. A record an earlier scan
left for one of them is deleted as unverified. README's smoke-test note
on keeping the database out of the tree is gone, as it no longer holds.

Judgement call: walkRules, in exclude.go, carries the noted files to the walk.

Model: opus-5-5

TODO.md

@@ -29,6 +29,13 @@
 
 # Completed Steps
 
+- `scan` never records its own database: once the database and its `.lock` file
+  are open, it notes the device and inode of the database file, its `-wal` and
+  `-shm` files and its `.lock` file, and the walk passes over a file with one of
+  them wherever the operands reach it, through a symlinked directory included,
+  printing no line and counting nothing. A record an earlier scan left for one
+  of them is deleted (2026-10-17, https://git.eeqj.de/sneak/sfdupes/issues/115)
+
 - `scan` never records or reads anything that is not on a persistent filesystem
   (`tmpfs`, `proc`, `sysfs` and the other kernel or memory-backed types),
   matched by the device ID of each such filesystem, listed as the scan starts