fix(data): remove MinMaxPyramid::reduce, repoint its two test callers

The pyramid's public query surface is the bucket-grained trio
(reduce_buckets/sum_buckets/sumsq_buckets); reduce() was the one
non-*_buckets query and silently over-covered non-aligned ranges, which
is exactly the misuse the exactness split exists to prevent. Exact range
queries stay on Series.

- delete the declaration, its doc paragraph and the definition; no
  helper became dead (fold_range/cover_buckets/ceil_to_bucket are all
  still used by the *_buckets queries)
- pyramid_clear_and_rebuild: complete bucket 0 and assert its extrema via
  reduce_buckets(0, kBucket) -- a pyramid has no exact sub-bucket query
- stream_series_acquire_publishes_snapshot: s.pyramid().reduce(0, 100)
  -> s.reduce(0, 100), keeping the == 99.0 assertion

tests/test_series.cpp

@@ -29,7 +29,7 @@ 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);