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

One ZFS commit can take a file through several sizes, and unix.Sync can
return before the blocks are counted. A test that expected the size on
disk to be settled therefore 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. It then rescans until the size reads the same before and
after a rescan, and compares disk_size exactly, within the same 30
seconds. The other two tests fail only on a disk_size outside their two
reads. They call unix.Sync once first, so a wrong 512 usually fails them.

Model: opus-5-5

TODO.md

@@ -29,6 +29,14 @@
 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, and one ZFS
+  commit can take a file through several sizes. 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. The rewrite test rescans until the two reads match, then
+  compares exactly. The other two fail only on a `disk_size` outside the two
+  reads (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