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.go

@@ -9,12 +9,16 @@ package bt
 import (
 	"context"
 	"fmt"
+	"io"
+	"log/slog"
+	"os"
 	"path/filepath"
 	"strconv"
 	"strings"
 	"sync/atomic"
 	"time"
 
+	alog "github.com/anacrolix/log"
 	missinggo "github.com/anacrolix/missinggo/v2"
 	"github.com/anacrolix/torrent"
 	"github.com/anacrolix/torrent/metainfo"
@@ -73,6 +77,14 @@ func ParsePorts(spec string) []int {
 	return ports
 }
 
+// quietSlogger filters anacrolix's structured logging down to Error and above,
+// so the routine webseed/peer/tracker warning chatter (the 403/429 noise) no
+// longer interleaves with — and corrupts — the live progress display. Genuine
+// errors still reach w.
+func quietSlogger(w io.Writer) *slog.Logger {
+	return slog.New(slog.NewTextHandler(w, &slog.HandlerOptions{Level: slog.LevelError}))
+}
+
 // NewClient builds the shared anacrolix client from the run-wide config. When a
 // listen-port spec is given it tries each candidate port in turn, advancing past
 // any that is already in use, and falls back to an OS-chosen port if none bind,
@@ -91,6 +103,11 @@ func ParsePorts(spec string) []int {
 //     prevention must come from a run-wide setting (e.g. --enable-dht=false).
 func NewClient(cfg ClientConfig) (*torrent.Client, error) {
 	c := torrent.NewDefaultClientConfig()
+	// Keep the library's own logging out of the readout. Webseed/peer warnings go
+	// through slog, the rest through the legacy analog logger, so filter both to
+	// Error and above (the analog default is Warning, which is what leaks today).
+	c.Slogger = quietSlogger(os.Stderr)
+	c.Logger = alog.Default.FilterLevel(alog.Error)
 	c.DataDir = cfg.Dir
 	if c.DataDir == "" {
 		c.DataDir = "."