compute: barrier discipline for MV sink shared batches - #37617

Wire the shared-batch builder into the sync (v2) materialized-view sink so
the workers in one process coalesce their parts for a given batch interval
into a single, larger batch instead of each writing its own small batch.

All workers building an interval share the batch description broadcast by the
mint operator, so they share a batch id and their parts land in one
process-global shared batch. Only the last worker to finish receives that
batch; the rest get nothing, which maps to the empty-batch response the write
operator already handles. Every worker still finishes its builder even when it
pushed no data, since any one of them may be the last holder responsible for
delivering all workers' parts.

The behavior is gated behind the new enable_compute_sync_mv_sink_shared_batches
dyncfg, default off. The prior per-worker path is retained unchanged for the
off case. Measured locally on an 8-worker replica with a churning view: blob
PUTs down ~67% and consensus state-diff bytes down ~54%, at unchanged consensus
command count.

Adds a testdrive test asserting the view equals the equivalent one-shot
aggregation across a multi-worker cluster with churn, enables the flag in the
CI system-parameter defaults, and registers it with parallel-workload's flag
flipper.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

Both MV sinks read `enable_compute_sync_mv_sink_shared_batches` through a
config handle when they start a batch, so a flip applies without re-rendering
the sink. The sync sink's shared and per-worker write paths are separate
functions.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

The flag now serves as a kill switch. Both sinks read it per batch
interval, so turning it off takes effect without re-rendering.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Production picks up shared batches through a staged parameter rollout
instead of on merge. CI keeps covering the path, since mzcompose sets the
flag to true and parallel workload toggles it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

The flag gates both the async and the sync MV sink, so its name should not
refer to the sync sink only.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Remove `enable_compute_mv_sink_shared_batches`. A zero
`compute_mv_sink_shared_batch_max_bytes` already disables sharing, so the
threshold now defaults to zero and is the only knob. This removes the
combination of an enabled flag with a zero threshold.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Best-effort coalescing only merges workers whose builder handles overlap in
time, so a process still writes ~1.4 parts per append. Add an opt-in barrier
that makes the process-local workers agree on a single batch per interval.

Each of the workers_per_process local workers reports exactly once per batch
id, either by a write (SharedBatchBuilder::finish) or, when it supersedes the
description without writing, by a skip (SharedBatches::note_skip from the
Timely thread). The shared batch is finished only after every participant
reports, and is delivered to the last worker that pushed data, which always
holds an output capability.

The wait has no timeout, since there is no cross-worker clock to base one on.
In steady state every local worker eventually writes or supersedes each
broadcast description, so the barrier completes. Teardown aborts the write
tasks. The batch is finished only after all participants have pushed, so no
push can race a finished builder.

The barrier is per process, because SharedBatches is process-local. A
multi-process replica coalesces to one batch per process, which the append
operator combines into a single compare_and_append.

The barrier is gated behind enable_compute_sync_mv_sink_shared_batches_barrier,
default off, and not randomized across the CI suite, because a missed report
would hang the sink. The shared-batches testdrive (single- and multi-process)
and the parallel-workload flag flipper cover it. On an 8-worker replica, parts
per append drop from ~1.4 to 1.0, cutting blob PUTs a further ~30%.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>