Replace the Rollup + `rollup-plugin-typescript2` build with a split
pipeline: tsdown (rolldown) bundles the JS in ~0.2s, and plain `tsc`
emits the declarations to `dist/types` (~2s). Full cold build drops from
~9.2s to ~2.5s (~3.6x) with zero source changes.
Unlike data-schemas, the fast oxc/isolated-declarations dts path isn't
viable here: the package's 78 exported zod schemas produce 374
`isolatedDeclarations` errors (TS9013/TS9038) and a `z.ZodType<T>`
annotation would break the 76 downstream `.extend`/`.shape`/`.pick`
usages. Plain `tsc` keeps the rich zod types intact, and since dts was
never the bottleneck (rollup-plugin-typescript2 was), the win stands.
- dts stays unbundled in `dist/types/` — identical to the prior output,
so the existing deep `dist/types` imports and the exports `types`
paths are unchanged.
- ESM output renamed `index.es.js` -> `index.mjs` (via the exports map;
no consumer hardcodes the old path). cjs/types paths unchanged.
- `./react-query` now emits a real cjs build + types — the exports map
already promised them, but Rollup only ever built the esm file.
- Kept `rollup` + the plugins used by `server-rollup.config.js`
(the `rollup:api` server-bundle smoke test in backend-review.yml);
removed only the deps used solely by the deleted `rollup.config.js`.
- Repointed CI build-cache keys from `rollup.config.js` to
`tsdown.config.mjs`.
