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.

go.mod

@@ -9,8 +9,12 @@ require (
 )
 
 require (
+	github.com/andybalholm/brotli v1.2.1 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/klauspost/compress v1.18.6 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/stretchr/objx v0.5.2 // indirect
+	github.com/valyala/bytebufferpool v1.0.0 // indirect
+	github.com/valyala/fasthttp v1.71.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )