feat(client): opt-in fasthttp transport (NewFastHTTPDoer)

Adds an alternative HTTPDoer backed by valyala/fasthttp for high-throughput
bots. Cuts per-call allocs from 102 to 56 in the cross-library bench
(within 8 of telego, which uses fasthttp by default), and per-call bytes
from 11.1 KiB to 6.6 KiB.

  bot := client.New(token,
      client.WithHTTPClient(client.NewFastHTTPDoer()),
  )

Implementation notes:
  - Wraps *fasthttp.Client behind the existing HTTPDoer (Do *http.Request)
    interface, so RetryDoer, custom transports, observability middleware,
    and the 1428 generated tests all keep working as-is.
  - Translates *http.Request -> fasthttp.Request once per call and
    returns a *http.Response whose Body releases the pooled fasthttp
    response on Close (net/http contract).
  - Recognises the bufferReadCloser / readerReadCloser shapes produced
    by buildRequest and passes their underlying bytes straight to
    SetBodyRaw -- no io.ReadAll, no copy.
  - Honours ctx.Deadline via DoDeadline, falls back to WithFastHTTPReadTimeout
    when no deadline is set. fasthttp.ErrTimeout maps to
    context.DeadlineExceeded for errors.Is compatibility.

Default stays net/http: fasthttp is HTTP/1.1 only, doesn't compose with
the http.RoundTripper middleware ecosystem, and most users don't have
the throughput to notice. Bots making thousands of API calls/sec should
opt in.

Multipart/file-upload path remains on net/http per the agreed scope --
the perf bottleneck was JSON-method round-trip, not file uploads.

Time numbers in the report deferred until a quiet-system bench run;
allocs/bytes numbers (which are deterministic per code path) are
already updated.

README.md

@@ -328,9 +328,12 @@ Apples-to-apples micro-benchmarks against the five most-starred Go Telegram libr
 |------|---------|--------------|
 | Webhook decode (small Update) | **ours** — 1.83 µs / 11 allocs | 1st of 6 |
 | Large Update unmarshal (unions + reply markup) | **ours** — 6.73 µs / 34 allocs | 1st of 6 |
-| `sendMessage` round-trip (mock server) | telego — 35.8 µs / 48 allocs | 2nd of 5 |
+| `sendMessage` round-trip — `net/http` default | telego — 35.8 µs / 48 allocs | 2nd of 5 (102 allocs) |
+| `sendMessage` round-trip — opt-in `fasthttp` | telego — 48 allocs | within 8 of telego (56 allocs) |
 | Dispatcher routing (20 handlers, last matches) | **ours** — 98 ns / 3 allocs | 1st of 3 |
 
+Opt into fasthttp for high-throughput bots: `client.WithHTTPClient(client.NewFastHTTPDoer())`. Trade-off: HTTP/1.1 only, no `RoundTripper` middleware composition.
+
 Full tables, caveats, and reproduction steps: **[`docs/benchmarks/2026-05-10-comparison.md`](docs/benchmarks/2026-05-10-comparison.md)**.
 
 </details>