compute: consolidate operator output only while it recovers records

[frankmcsherry]

Several render operators emit through differential's ConsolidatingContainerBuilder, which sorts and consolidates every output chunk of about a thousand records before it leaves the operator. That collapses repeated (data, time) pairs before they are exchanged or arranged, a large saving when the data has few distinct keys. When it has many, the sort recovers nothing: a GROUP BY over 1.25M groups spread across 10M rows merges under one percent of each chunk, and the sort was the largest cost of the reduce's key formation and of the accumulator explosion.

AdaptiveConsolidatingContainerBuilder (in mz-timely-util) keeps differential's buffering and measures what each consolidation recovers, as an exponential moving average in permille. While that stays below two percent it passes chunks through unsorted, consolidating one chunk in every thirty-two so a change in the data is noticed within that many chunks. Consumers never relied on the output being consolidated or ordered, so nothing downstream changes.

It replaces the differential builder at four sites: the reduce's key formation, explode_one (the accumulable reduce's accumulator explosion), the MFP flat_map on collections, and the FlatMap operator's Ok output. Error outputs and the join closures keep the differential builder.

Hydration at 10M rows, one worker, median of three, on a local build with the other hydration PRs in flight applied:

shape	before	after	operators
SELECT k % g, count(*), sum(v) ... GROUP BY 1 (1.25M groups)	4.42s	3.81s	ExplodeOne 919 to 514ms, CollectionFlatMap 1233 to 968ms, ArrangeAccumulable 1073 to 1097ms
max(v) by k % g with AGGREGATE INPUT GROUP SIZE = 8	4.50s	4.28s	CollectionFlatMap 1166 to 916ms
SELECT DISTINCT k, v	3.01s	2.93s
index on a table	2.00s	1.95s

The downstream arrangements pay slightly more for unconsolidated input (ArrangeAccumulable +24ms); the sorts they no longer need paid far more.

Unit tests cover the three regimes: repeated keys stay consolidated, distinct keys stop the sort without losing records, and a probe resumes consolidation when keys start repeating.