Migrate off the deprecated `nats` package to the `@nats-io/*` scoped v3 client — swap `nats@2` for `@nats-io/transport-node@3` across all 10 services, drop the removed `JSONCodec` in favor of `JSON.stringify` + `msg.json()`, and prove the inter-service bus still round-trips live before committing

The monolithic `nats` package (we're on `^2.28.0`) is fully deprecated — pnpm flags it on every install. nats.js v3 split the library into scoped `@nats-io/*` packages; for our pure-core usage the whole bus moves to `@nats-io/transport-node@3.4.0`. The surface is small and uniform — `connect`, `subscribe` (84×), `publish` (46×), `request` (30×), and `JSONCodec` encode/decode across 27 files in 10 services, with zero JetStream, KV, queue groups, or headers — so the migration is mechanical, but it touches every service's message plumbing at once, and the one real API change (the removed codec) rewrites how every message is encoded and decoded. This plan gives the next session an exact old→new mapping, a one-service pilot that proves the round-trip on a live bus, a per-service migration checklist, and a full-stack verification gate. The NATS server itself is untouched — this is a client-library swap; the wire protocol is unchanged.