feat(seeding): wire seed ratio/time lifecycle into the torrent daemon

SeedRatio/SeedTime were declared on TorrentConfig but never consumed, and
SeedEnabled was hardcoded false in both constructors — the daemon never
seeded, and if forced it seeded forever.

- config: [downloads] seed_enabled/seed_ratio/seed_time (opt-in, off by default)
- daemon: parse seed_time + wire all three; startup log per target shape
- engine: seedTargetReached() (pure) + seedAndDrop() background monitor on a
  downloader-scoped seedCtx (not the task ctx, which dies when Download returns);
  drops the torrent on ratio (uploaded/size) OR time, whichever first; no target
  = seed until shutdown. Configurable check interval (tests lower it).
- fix: cleanup() now always drops — previously leaked the handle on error paths
  when seeding was enabled.
- refactor: dropTracked() helper shared by cleanup + post-seeding drop.

Tests: TestSeedTargetReached (9 cases) + ctx/no-target branches + loopback
swarm smoke (-tags smoke). Roadmap hueco closed.

internal/engine/seed_lifecycle_smoke_test.go

@@ -0,0 +1,118 @@
+//go:build smoke
+
+package engine
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/anacrolix/torrent"
+	"github.com/anacrolix/torrent/bencode"
+	"github.com/anacrolix/torrent/metainfo"
+)
+
+// TestSeedLifecycleSmoke spins up a real loopback BitTorrent swarm: a seeder
+// client serving a small file, and our TorrentDownloader's client leeching it.
+// Once the leecher completes, the torrent is handed to seedAndDrop with a short
+// SeedTime; the test asserts the lifecycle fires and the handle is dropped
+// (removed from d.active). Exercises the real anacrolix Stats/Drop/ticker path,
+// not mocks. Run with: go test -tags smoke -run TestSeedLifecycleSmoke ./internal/engine/
+func TestSeedLifecycleSmoke(t *testing.T) {
+	// --- seeder: a real client serving a 4 MiB file over loopback ---
+	seedDir := t.TempDir()
+	payload := make([]byte, 4<<20)
+	for i := range payload {
+		payload[i] = byte(i)
+	}
+	if err := os.WriteFile(filepath.Join(seedDir, "movie.bin"), payload, 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	var info metainfo.Info
+	info.PieceLength = 256 << 10
+	if err := info.BuildFromFilePath(filepath.Join(seedDir, "movie.bin")); err != nil {
+		t.Fatalf("build info: %v", err)
+	}
+	var mi metainfo.MetaInfo
+	var err error
+	if mi.InfoBytes, err = bencode.Marshal(info); err != nil {
+		t.Fatalf("marshal info: %v", err)
+	}
+
+	scfg := torrent.NewDefaultClientConfig()
+	scfg.DataDir = seedDir
+	scfg.Seed = true
+	scfg.NoDHT = true
+	scfg.DisableTrackers = true
+	scfg.ListenPort = 0 // random — never collides with the leecher's 42069
+	seeder, err := torrent.NewClient(scfg)
+	if err != nil {
+		t.Fatalf("seeder client: %v", err)
+	}
+	defer seeder.Close()
+	st, err := seeder.AddTorrent(&mi)
+	if err != nil {
+		t.Fatalf("seeder add: %v", err)
+	}
+	<-st.GotInfo()
+	st.DownloadAll() // verifies the existing pieces so the seeder is "complete"
+
+	// --- leecher: our downloader, seeding enabled, very short seed time ---
+	leechDir := t.TempDir()
+	dl, err := NewTorrentDownloader(TorrentConfig{
+		DataDir:     leechDir,
+		SeedEnabled: true,
+		SeedTime:    1 * time.Second, // time target fires fast (no peers pull from us, so ratio stays 0)
+	})
+	if err != nil {
+		t.Fatalf("downloader: %v", err)
+	}
+	dl.seedCheckInterval = 200 * time.Millisecond // poll fast so the 1s target is noticed promptly
+	defer dl.Shutdown(context.Background())
+
+	lt, err := dl.client.AddTorrent(&mi)
+	if err != nil {
+		t.Fatalf("leecher add: %v", err)
+	}
+	<-lt.GotInfo()
+	lt.AddClientPeer(seeder) // loopback peer — no DHT/tracker needed
+	lt.DownloadAll()
+
+	deadline := time.After(30 * time.Second)
+	for lt.BytesMissing() > 0 {
+		select {
+		case <-deadline:
+			t.Fatalf("download did not complete (missing %d bytes)", lt.BytesMissing())
+		case <-time.After(100 * time.Millisecond):
+		}
+	}
+	t.Logf("leecher completed %d bytes", lt.BytesCompleted())
+
+	// Track it as the daemon would for a seeding torrent, then run the lifecycle.
+	const taskID = "smoke-seed-task-0001"
+	dl.activeMu.Lock()
+	dl.active[taskID] = lt
+	dl.activeMu.Unlock()
+
+	done := make(chan struct{})
+	go func() {
+		dl.seedAndDrop(taskID, lt, info.Length)
+		close(done)
+	}()
+
+	select {
+	case <-done:
+	case <-time.After(10 * time.Second):
+		t.Fatal("seedAndDrop did not return within 10s")
+	}
+
+	dl.activeMu.Lock()
+	_, stillTracked := dl.active[taskID]
+	dl.activeMu.Unlock()
+	if stillTracked {
+		t.Error("torrent still tracked after seedAndDrop — lifecycle did not drop it")
+	}
+}