feat(counter): go-hua/single 迁入并更名 counter(键值计数器)

- 原包名 single 名不达意，实为 string→int64 并发计数器；类型 Single→Counter，
  方法签名不变(Init/Get/Incr/Set)，附测试、例程与 README

counter/counter_test.go

@@ -0,0 +1,57 @@
+package counter
+
+import (
+	"sync"
+	"testing"
+)
+
+func TestIncrConcurrent(t *testing.T) {
+	var c Counter
+	c.Init()
+
+	var wg sync.WaitGroup
+	for i := 0; i < 100; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			c.Incr("pv", 1)
+		}()
+	}
+	wg.Wait()
+
+	if got := c.Get("pv"); got != 100 {
+		t.Errorf("并发 100 次 Incr 后 = %d, want 100", got)
+	}
+}
+
+func TestSet(t *testing.T) {
+	var c Counter
+	c.Init()
+
+	c.Set("k", 10)
+	old := c.Set("k", 20)
+	if old != 10 {
+		t.Errorf("Set 返回旧值 = %d, want 10", old)
+	}
+	if got := c.Get("k"); got != 20 {
+		t.Errorf("Set 后 Get = %d, want 20", got)
+	}
+}
+
+func TestGetMissing(t *testing.T) {
+	var c Counter
+	c.Init()
+	if got := c.Get("not-exist"); got != 0 {
+		t.Errorf("不存在的 key Get = %d, want 0", got)
+	}
+}
+
+func TestKeysIndependent(t *testing.T) {
+	var c Counter
+	c.Init()
+	c.Incr("a", 1)
+	c.Incr("b", 2)
+	if c.Get("a") != 1 || c.Get("b") != 2 {
+		t.Errorf("不同 key 应相互独立: a=%d b=%d", c.Get("a"), c.Get("b"))
+	}
+}