perf(zero-cache): batch replication write-worker messages

[Karavil]

what changed

Adds write-worker message batching and keeps legacy commit-boundary flushing for the existing subscribe path.

benchmark output

Targeted run on this branch, medium-wide-batch-pressure, 15s:

worker-message tx/s: 946.6
worker-batch tx/s: 969.9
worker-message rows/s: 18,932.1
worker-batch rows/s: 19,397.4
avg VS tx apply: 0.731 ms -> 0.585 ms

validation

pnpm exec vitest --project='*no-pg*' run src/services/replicator/worker-message-batcher.test.ts src/services/replicator/write-worker.test.ts bench/rm-vs-load/config.test.ts bench/rm-vs-load/scenarios.test.ts bench/rm-vs-load/workloads.test.ts
pnpm run check-types
pnpm exec oxlint --config ../../oxlint.config.ts src/services/replicator/worker-message-batcher.ts src/services/replicator/worker-message-batcher.test.ts src/services/replicator/write-worker.ts

[Karavil]

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