fix: 集群操作日志同步修复 — 全量回填 + 重启节点 seq 水位抬升

两个根因:
1. 离线错过条目永久缺失: token delta 被下游 trim (keep=seq>wm)，
   离线节点错过的 seq 再也收不到 → 'log delta gap: want N got N+1' 死循环。
   修复: OnToken 改为附带自己的全量日志 (Snapshot)，接收方 ApplyDelta 按 seq 幂等去重，
   缺口节点下一轮自动补齐。日志量小(几十条)，开销可忽略。

2. 重启节点重新从 seq=1 编号: NewClusterLog 从 Seq=0 起，重启后产生的新事件
   与环上历史 seq 冲突 → ApplyDelta 视为 already-have 静默丢弃 + 全量附带出现歧义 id。
   修复: OnToken 收到日志时把本地 Seq 抬到环高水位之上，新编号接在历史之后。

测试: TestFullLogBackfillAfterGap / TestApplyDeltaIdempotentOnFullResend

实测: 三台集群撤销 minecraft 转发 → 三台均记录 seq=10 forward.remove;
恢复后三台均记录 seq=11 forward.add

internal/cluster/ring_log_test.go

@@ -77,3 +77,60 @@ func TestClaimLogsToEngine(t *testing.T) {
 		t.Fatalf("engine log = %+v", snap)
 	}
 }
+
+// TestFullLogBackfillAfterGap: a node that missed entries while offline
+// (Synced stuck below the ring's max) must backfill from a peer's FULL log
+// attachment. This is the regression test for the "log delta gap: want 1 got
+// N" livelock where offline nodes could never rejoin the log history.
+func TestFullLogBackfillAfterGap(t *testing.T) {
+	// Peer with complete history [1..4].
+	var peer ClusterLog
+	for i := 1; i <= 4; i++ {
+		if _, err := peer.Append("n1", LogNodeJoin, map[string]int{"i": i}); err != nil {
+			t.Fatal(err)
+		}
+	}
+	// Straggler that only has [1]; it missed [2..3] while offline and now
+	// receives the peer's FULL attachment [1..4].
+	straggler := NewClusterLog()
+	if _, err := straggler.Append("n2", LogNodeJoin, nil); err != nil {
+		t.Fatal(err)
+	}
+	// Force straggler to look like it has seq1 only (Synced=1).
+	straggler.Synced = 1
+
+	wm, err := straggler.ApplyDelta(peer.Snapshot())
+	if err != nil {
+		t.Fatalf("full backfill failed: %v", err)
+	}
+	if wm != 4 {
+		t.Fatalf("watermark = %d, want 4", wm)
+	}
+	if len(straggler.Snapshot()) != 4 {
+		t.Fatalf("log length = %d, want 4 (no dupes)", len(straggler.Snapshot()))
+	}
+}
+
+// TestApplyDeltaIdempotentOnFullResend: applying the same full attachment
+// twice must not duplicate entries or error — peers re-attach their full log
+// every cycle now.
+func TestApplyDeltaIdempotentOnFullResend(t *testing.T) {
+	var src ClusterLog
+	for i := 1; i <= 3; i++ {
+		if _, err := src.Append("n1", LogNodeJoin, nil); err != nil {
+			t.Fatal(err)
+		}
+	}
+	full := src.Snapshot()
+	dst := NewClusterLog()
+	if _, err := dst.ApplyDelta(full); err != nil {
+		t.Fatalf("first apply: %v", err)
+	}
+	wm, err := dst.ApplyDelta(full)
+	if err != nil {
+		t.Fatalf("second apply (idempotence): %v", err)
+	}
+	if wm != 3 || len(dst.Snapshot()) != 3 {
+		t.Fatalf("wm=%d len=%d, want 3/3", wm, len(dst.Snapshot()))
+	}
+}