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_minmax_pyramid.cpp

@@ -103,7 +103,13 @@ 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);
+  // A pyramid can only answer exactly for whole buckets, so complete the first
+  // bucket and ask the bucket-grained query for its extrema.
+  for (u32 i = 1; i < kBucket; ++i) p.append(1.0f);
+  PLOT_CHECK(p.size() == kBucket);
+  PLOT_CHECK(p.level(0).size() == 1);
+  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);
 }
 
 PLOT_TEST(pyramid_cover_cost_is_logarithmic_for_unaligned_starts) {