Add async HTTP client (curl_multi + ASIO) (#918)

- Adds `AsyncHttpClient` — an asynchronous HTTP client using `curl_multi_socket_action` integrated with ASIO for event-driven I/O. Supports GET, POST, PUT, DELETE, HEAD with both callback-based and `std::future`-based APIs.
- Extracts shared curl helpers (callbacks, URL encoding, header construction, error mapping) into `httpclientcurlhelpers.h`, eliminating duplication between the sync and async implementations.

## Design

- All curl_multi state is serialized on an `asio::strand`, safe with multi-threaded io_contexts.
- Two construction modes: owned io_context (creates internal thread) or external io_context (caller runs the loop).
- Socket readiness is detected via `asio::ip::tcp::socket::async_wait` driven by curl's `CURLMOPT_SOCKETFUNCTION`/`CURLMOPT_TIMERFUNCTION` — no polling, sub-millisecond latency.
- Completion callbacks are dispatched off the strand onto the io_context so slow callbacks don't starve the curl event loop. Exceptions in callbacks are caught and logged.

## Files

| File | Change |
|------|--------|
| `zenhttp/include/zenhttp/asynchttpclient.h` | New public header |
| `zenhttp/clients/asynchttpclient.cpp` | Implementation (~1000 lines) |
| `zenhttp/clients/httpclientcurlhelpers.h` | Shared curl helpers extracted from sync client |
| `zenhttp/clients/httpclientcurl.cpp` | Removed duplicated helpers, uses shared header |
| `zenhttp/asynchttpclient_test.cpp` | 8 test cases: verbs, payloads, callbacks, concurrency, external io_context, connection errors |
| `zenhttp/zenhttp.cpp` | Forcelink registration for new tests |

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