Add Hono example (and decide on Web Headers support)

[gmoon]

After v4.0 lands (#103), add a Hono example to demonstrate that
`proxy.fetch()` works cleanly with Web Standards runtimes
(Bun, Workers, Deno) where Hono is the de facto framework.

Why

The v4 `proxy.fetch()` API was designed pure (no `res` mutation)
specifically so frameworks built around `Web Response` could use it.
Hono is the obvious validation case. The README's "Framework
Compatibility" list doesn't currently include Hono; the example would
back it.

Sketch

```ts
import { Hono } from 'hono';
import { S3Proxy } from 's3proxy';
import type { HttpRequest } from 's3proxy';

const proxy = new S3Proxy({ bucket: 's3proxy-public' });
await proxy.init();

const app = new Hono();

app.get('/health', async (c) => {
await proxy.healthCheck();
return c.text('OK');
});

app.get('*', async (c) => {
const url = new URL(c.req.url);
const { stream, status, headers } = await proxy.fetch({
url: url.pathname + url.search,
method: c.req.method,
headers: Object.fromEntries(c.req.raw.headers),
} as HttpRequest);
return new Response(stream as unknown as ReadableStream, { status, headers });
});

export default app;
```

The decision point

Hono passes a `Web Request` whose `.headers` is a `Headers`
object (iterable, not a `Record`). The example above adapts via
`Object.fromEntries(headers)`. That works, but it's friction.

Two paths:

A. Document the adapter pattern in the example. Cheap. The
adapter is one line. Hono users copy-paste it once. The library stays
focused on `Record<string, string | string[]>`-shaped headers.

B. Widen `HttpRequest['headers']` to accept `Headers | Record<...>`,
and have `mapHeaderToParam` handle both. More invasive — the parser
and any extension package now has two header shapes to think about.

I'd lean (A) unless the friction shows up in more than just Hono. If
SvelteKit / Remix / Vercel functions all need similar adapters,
(B) starts to look right.

Definition of done

• `examples/hono-basic.ts` running via `tsx` against `s3proxy-public`.
• Smoke gate (`scripts/smoke-examples.sh`) extended to cover it.
• README "Framework Compatibility" list adds Hono with a ✅.
• A short paragraph in README documenting the adapter pattern.
• If we go with (B), follow-up PR widens the type and updates parser tests.