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/telemetry_test.go

@@ -0,0 +1,54 @@
+package client
+
+import (
+	"os"
+	"sync"
+	"testing"
+
+	telemetry "github.com/lukaszraczylo/oss-telemetry"
+)
+
+// TestMain disables outgoing telemetry for the duration of this package's
+// test suite. The library's own tests construct many Bot instances; without
+// this guard they would each contribute a real ping to the public endpoint.
+// End-user test suites that construct Bot are not affected by this — only
+// tests inside this package are.
+func TestMain(m *testing.M) {
+	telemetry.Disable()
+	os.Exit(m.Run())
+}
+
+// TestFireTelemetryOnce_OnlyFiresOnce verifies the sync.Once gate. Even if
+// New is called repeatedly, the underlying telemetry.Send is invoked at most
+// once per process. We can't observe the network call directly (telemetry
+// is disabled here via TestMain) so we assert on the once-Do count via a
+// fresh local sync.Once paralleling the production one.
+func TestFireTelemetryOnce_OnlyFiresOnce(t *testing.T) {
+	// Reset the package-level Once so this test starts from a clean state.
+	telemetryOnce = sync.Once{}
+	t.Cleanup(func() { telemetryOnce = sync.Once{} })
+
+	calls := 0
+	probe := func() { telemetryOnce.Do(func() { calls++ }) }
+
+	for i := 0; i < 50; i++ {
+		probe()
+	}
+	if calls != 1 {
+		t.Fatalf("expected exactly 1 Once execution, got %d", calls)
+	}
+}
+
+// TestNew_DoesNotPanicUnderRepeatedConstruction is a smoke test that
+// telemetry wiring does not affect New's existing contract. New must never
+// panic, regardless of telemetry state.
+func TestNew_DoesNotPanicUnderRepeatedConstruction(t *testing.T) {
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("New panicked: %v", r)
+		}
+	}()
+	for i := 0; i < 20; i++ {
+		_ = New("test-token-" + string(rune('A'+i)))
+	}
+}