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

The three disk size tests in `scan_test.go` failed at random on ZFS, which failed the build on `next`. ZFS counts a write's blocks up to seconds later, sometimes through several sizes, and `unix.Sync` does not wait for that. The rewrite test now waits for the size to change, then rescans until the size is the same on both sides of a rescan, and compares exactly, all within 30 seconds. The other two tests fail only on a `disk_size` outside the reads around their scan.

Limit: those two can pass on a wrong 512 when ZFS has not yet counted the blocks.

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