Say what the fuzzers can no longer tell about the stale-answer guard

Dropping `dirty_size_under` from it now passes every run there is. It stays
for the one entry the bottom-up ordering does not reach -- `update` draws
the root for a resize before `redraw_updates` runs -- which is a hole
reasoned rather than measured, and the note says which.

core/src/ui/render_state.rs

@@ -202,6 +202,14 @@ impl UiRenderState {
         // that marks a reader is in `redraw`, which is not what asked here.
         // Both retained routes are an answer, so the question is asked once
         // rather than by each of them.
+        //
+        // Since `a92c6ac` settles a frame strictly bottom-up, no fuzzer can
+        // tell whether the second half of this still does anything: dropping
+        // `dirty_size_under` passes the suite, the shrinker at 400 seeds of
+        // depth 5, the oracle at 1000 of depth 6 and 2000 seeds at depth 4.
+        // It stays because `update` draws the root for a resize before
+        // `redraw_updates` runs at all, which that ordering does not reach --
+        // a hole that is reasoned rather than measured.
         let stale =
             rsc.widgets().needs_redraw.contains(&id) || self.dirty_size_under(id, rsc.widgets());
         let replace_answer = self.answer_invalid.remove(&id) || (self.replace_answers && stale);