feat: anonymous adoption telemetry on first client.New

Sends one fire-and-forget POST per process the first time a consumer
constructs a Bot via client.New. Helps track real-world adoption and
version spread of the library. No identifiers, no API contents.

Implementation:
- new client/version.go: exported Version var (currently 0.7.11)
- new client/telemetry.go: sync.Once gate + telemetry.Send call
- client.go New(): single fireTelemetryOnce() line at function entry
- client/telemetry_test.go: TestMain disables outgoing pings during the
  library's own test suite

README §Telemetry documents the payload, the opt-out env vars, and links
to the upstream oss-telemetry source so consumers can audit what is sent.

Opt out via any of:
  DO_NOT_TRACK=1
  OSS_TELEMETRY_DISABLED=1
  GO_TELEGRAM_DISABLE_TELEMETRY=1
  osstelemetry.Disable() before the first client.New

client/client.go

@@ -45,6 +45,7 @@ func (b *Bot) Logger() Logger { return b.logger }
 // NewDefaultHTTPDoer); the default codec wraps encoding/json; the default
 // logger discards records.
 func New(token string, opts ...Option) *Bot {
+	fireTelemetryOnce()
 	b := &Bot{
 		token:  token,
 		base:   defaultBaseURL,