docs: repoint plan call sites off the removed MinMaxPyramid::reduce

docs/superpowers/plans/2026-09-27-plot-library-scaffold.md

@@ -585,7 +585,9 @@ PLOT_TEST(pyramid_clear_and_rebuild) {
   PLOT_CHECK(p.level_count() <= 1);
   p.append(7.0f);
   PLOT_CHECK(p.size() == 1);
-  PLOT_CHECK_NEAR(p.reduce(0, 1).max, 7.0, 1e-6);
+  for (int i = 1; i < int(kBucket); ++i) p.append(1.0f);   // complete bucket 0
+  PLOT_CHECK_NEAR(p.reduce_buckets(0, kBucket).max, 7.0, 1e-6);
+  PLOT_CHECK_NEAR(p.reduce_buckets(0, kBucket).min, 1.0, 1e-6);
 }
 ```
 
@@ -643,7 +645,7 @@ PLOT_TEST(stream_series_acquire_publishes_snapshot) {
   s.acquire();
   PLOT_CHECK(s.size() == 100);
   // Pyramid must track the published samples.
-  PLOT_CHECK_NEAR(s.pyramid().reduce(0, 100).max, 99.0, 1e-4);
+  PLOT_CHECK_NEAR(s.reduce(0, 100).max, 99.0, 1e-4);
   for (int i = 100; i < 250; ++i) s.push(f32(i));
   s.acquire();
   PLOT_CHECK(s.size() == 250);