Stop the disk size tests failing at random on ZFS (closes #150)

unix.Sync can return before ZFS has counted a file's blocks, so a test
that expected the size on disk to be settled after it failed at random.
The test that needs the size to change now calls unix.Sync in a loop
until it does, for at most 30 seconds. All three tests read the size
before and after the scan and accept either value.

Model: opus-5-5

TODO.md

@@ -29,6 +29,12 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- The three disk size tests in `scan_test.go` no longer fail at random on ZFS.
+  `unix.Sync` can return before ZFS has counted a file's blocks. The test that
+  needs the size to change waits for it, for at most 30 seconds. Each test reads
+  the size before and after its scan and accepts either value (2026-11-02,
+  https://git.eeqj.de/sneak/sfdupes/issues/150)
+
 - `TODO.md` rewritten from scratch by the same rules as `README.md`, sentence by
   sentence. Every completed step is kept, with its date and its issue or branch.
   The policy-compliance checklist of 2026-07-22 is folded into its step