perf(zero-cache): batch v6 websocket change frames

[Karavil]

what changed

Preserves the v6 Downstream payload shape while allowing websocket batch frames and subscribeBatched(). Incremental sync flushes the worker batch after a received websocket frame.

ACK semantics stay per-message in this PR.

benchmark output

medium-wide-batch-pressure, 15s, compared to hot queue helpers:

tx/s: 1,652.3 -> 2,099.4
rows/s: 33,046.2 -> 41,987.6
fanout msg/s: 51,531.8 -> 68,585.4
websocket messages: 807,248 -> 557
websocket ACKs: 807,248 -> 1,046,498
reconnect catchup to join: 1,658.3 ms -> 275.1 ms

The ACK count rises because this PR intentionally keeps old v6 per-message ACK behavior while batching frames.

validation

pnpm exec vitest --project='*no-pg*' run src/types/streams.test.ts src/services/change-streamer/subscriber.test.ts src/services/replicator/worker-message-batcher.test.ts bench/rm-vs-load/config.test.ts bench/rm-vs-load/scenarios.test.ts bench/rm-vs-load/workloads.test.ts
pnpm exec vitest --project='*15*' run src/services/change-streamer/change-streamer-http.pg.test.ts src/services/change-streamer/change-streamer-service.pg.test.ts src/services/change-streamer/storer.pg.test.ts
pnpm run check-types

[Karavil]

Replaced by rocicorp/mono PRs rocicorp#6070 through rocicorp#6078.