perf(zero-cache): apply serving replica changes locally

[Karavil]

what changed

Serving mode applies changes through a local write-worker client to avoid thread hops. Backup mode keeps the existing thread-worker path.

benchmark output

medium-wide-batch-pressure, 15s, compared to worker batching:

tx/s: 946.6 -> 1,836.4
rows/s: 18,932.1 -> 36,728.1
fanout msg/s: 31,512.8 -> 57,399.7
avg VS tx apply: 0.731 ms -> 0.398 ms

validation

pnpm exec vitest --project='*no-pg*' run src/workers/replicator.test.ts
pnpm run check-types
pnpm exec oxlint --config ../../oxlint.config.ts bench/rm-vs-load/e2e.ts src/services/replicator/local-write-worker-client.ts src/server/replicator.ts src/services/replicator/replicator.ts src/workers/replicator.ts src/workers/replicator.test.ts

[Karavil]

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