bt: filter anacrolix logs to errors so warnings don't corrupt the readout

webseed/peer/tracker warnings (the 403/429 chatter) go through slog, the
rest through the legacy analog logger; both default to Warning, which
interleaves with and corrupts the live progress block. Filter both to
Error and above so only genuine errors reach stderr. quietSlogger is a
small testable helper; anacrolix/log becomes a direct dependency.

bt/bt_test.go

@@ -1,10 +1,38 @@
 package bt
 
 import (
+	"bytes"
+	"context"
+	"log/slog"
 	"reflect"
+	"strings"
 	"testing"
 )
 
+// TestQuietSloggerDropsWarnings: the library's warning chatter (the webseed
+// 403/429 noise) is filtered out so it can't corrupt the live display, while a
+// genuine error still gets through.
+func TestQuietSloggerDropsWarnings(t *testing.T) {
+	var buf bytes.Buffer
+	lg := quietSlogger(&buf)
+
+	lg.Warn("webseed request error", "err", "429 Too Many Requests")
+	if buf.Len() != 0 {
+		t.Errorf("warning leaked into output: %q", buf.String())
+	}
+	// anacrolix's webseed code logs via Log(ctx, level, ...); confirm that path is
+	// filtered too, not just the Warn helper.
+	lg.Log(context.Background(), slog.LevelWarn, "another warning")
+	if buf.Len() != 0 {
+		t.Errorf("Log-at-Warn leaked: %q", buf.String())
+	}
+
+	lg.Error("real failure")
+	if !strings.Contains(buf.String(), "real failure") {
+		t.Errorf("error was suppressed, want it kept: %q", buf.String())
+	}
+}
+
 func TestParsePorts(t *testing.T) {
 	tests := []struct {
 		spec string