cluster / persist: Shared batches in the sink - #35411

Continues #35411 (draft by @bkirwi), rebased onto `main`.

With many workers each MV sink worker writes its own small batch per
interval, so the sink emits many small parts and a large consensus state
diff. This coalesces the parts the workers in one process write for a
batch interval into a single, larger shared batch.

`SharedBatches` hands out a `SharedBatchBuilder` per batch id. Handles
for the same id feed one process-global builder task, and the last
handle to finish receives the batch. Both MV sinks use it, best-effort.
On an 8-worker replica this cut persist blob PUTs ~67% and consensus
state-diff bytes ~54%, at unchanged consensus command count.

The single builder task serializes encoding and upload, which slowed
hydration of a 9.5 GB MV from 26 s to 33 s. Each worker therefore stages
its updates for an interval and only contributes them to the shared
batch while they stay below `compute_mv_sink_shared_batch_max_bytes` (1
MiB in these measurements). Past that, the worker writes its own batch,
and the append operator appends both kinds together. With the threshold,
hydration and 5M-row inserts match the per-worker path, while 1000-row
writes drop from ~7.8 to ~1.05 data parts per append. Writes small
enough for persist's inline path now cost one blob write per append
instead of none, at about +10 ms p50 latency and 2.5x fewer compaction
parts.

`compute_mv_sink_shared_batch_max_bytes` is the only knob. It defaults
to 0, which disables sharing, and production enables it through a staged
parameter rollout. CI runs it at several values, including 0. Both sinks
read it per batch interval, so a change applies without re-rendering the
sink.

Best-effort means a worker only merges into the shared batch when its
write overlaps the others still building in the same interval, so parts
per append land around 1.4, where the ideal is 1.0. The barrier that
closes that gap is split into the follow-up #37617.

The async (v1) and sync (v2) sinks share the batch-writing code in
`sink::shared_batch`. Unlike @bkirwi's draft, the v1 path is behind the
same flag, since v1 is the default sink.

Original authorship by @bkirwi is preserved in the first commit.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Ben Kirwin <[email protected]>
Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>