New optional driver under `misina/driver/undici`. Hands the entire
connection-pool / keep-alive / pipelining / HTTP/2 surface back to the
caller through any `undici.Agent` / `Pool` / `Client` they construct.
Solves the cross-region inference + RPC throughput problem the issue
flags: native fetch caps at 5 idle connections per origin, won't
negotiate H2, exposes no knobs.

```ts
import { Agent } from "undici"
import { undiciDriver } from "misina/driver/undici"

const api = createMisina({
  driver: undiciDriver({
    dispatcher: new Agent({
      connections: 100,
      keepAliveTimeout: 30_000,
      allowH2: true,
    }),
  }),
})
```

Implementation:
- `undici` is declared in `peerDependencies` with
  `peerDependenciesMeta.undici.optional: true`, so users who don't
  use this driver pay nothing. Range: `^6 || ^7 || ^8`.
- `undici` is **dynamically imported** the first time `request()`
  runs. The rest of misina keeps its static graph zero-dep.
- We re-issue the request as `fetch(url, { method, headers, body,
  signal, redirect, dispatcher })` instead of passing the `Request`
  instance directly — undici 8 stopped accepting cross-realm
  `Request` objects (nodejs/undici#4170).
  Bodies forward with `duplex: "half"` so streaming uploads work.
- `dispatcher` is typed as `any` on purpose — it's an opaque handle
  from undici, and pinning it to a specific class would force every
  consumer to align their undici major exactly. The driver doesn't
  introspect it, just forwards it.
- `fetch` override option lets tests bypass the dynamic import.

Bundle: `driver/undici.mjs` capped at 1.5KB (currently ~700B). Lives
outside the existing 6KB `driver/` budget so the figure stays
honest.

Tests in test/driver-undici.test.ts spin up a local `node:http`
fixture and verify:
- Real undici Agent routes the request, returns a structurally-
  compatible Response.
- A pool with `connections: 1` reuses sockets across three
  concurrent requests (bounded socket count).
- Headers + method survive the round-trip (uses an `/echo-headers`
  endpoint).
- The `fetch` override path is honored without touching `undici`.

`jsr.json` gets the matching `./driver/undici` export. README adds:
- A new highlight bullet.
- A "misina/driver/undici (Node-only, opt-in)" subsection under
  Drivers with the high-throughput recipe + a note that the rest
  of the package stays zero-dep.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>