fix(selector): safe type assertion, clean up debug logging, add doc comments and 64 tests

Replace bare type assertion in ParallelStrategy with comma-ok check to
prevent panic on non-Node inputs. Downgrade noisy Infof hash-selection
log to Tracef and remove Chinese-language debug output. Add doc comments
to all exported symbols in weighted.go.

selector/weighted_test.go

@@ -0,0 +1,130 @@
+package selector
+
+import (
+	"testing"
+)
+
+func TestRandomWeighted_Empty(t *testing.T) {
+	rw := NewRandomWeighted[int]()
+	if v := rw.Next(); v != 0 {
+		t.Fatalf("expected zero value, got %d", v)
+	}
+}
+
+func TestRandomWeighted_Single(t *testing.T) {
+	rw := NewRandomWeighted[int]()
+	rw.Add(42, 10)
+	for i := 0; i < 100; i++ {
+		if v := rw.Next(); v != 42 {
+			t.Fatalf("expected 42, got %d", v)
+		}
+	}
+}
+
+func TestRandomWeighted_EqualWeights(t *testing.T) {
+	rw := NewRandomWeighted[int]()
+	rw.Add(1, 1)
+	rw.Add(2, 1)
+	rw.Add(3, 1)
+
+	counts := make(map[int]int)
+	const n = 3000
+	for i := 0; i < n; i++ {
+		counts[rw.Next()]++
+	}
+
+	for _, item := range []int{1, 2, 3} {
+		if counts[item] < n/3/2 {
+			t.Fatalf("item %d underrepresented: %d/%d", item, counts[item], n)
+		}
+	}
+}
+
+func TestRandomWeighted_WeightedDistribution(t *testing.T) {
+	rw := NewRandomWeighted[string]()
+	rw.Add("heavy", 99)
+	rw.Add("light", 1)
+
+	heavyCount := 0
+	const n = 5000
+	for i := 0; i < n; i++ {
+		if rw.Next() == "heavy" {
+			heavyCount++
+		}
+	}
+
+	// ~99% should be heavy
+	if heavyCount < int(float64(n)*0.95) {
+		t.Fatalf("heavy underrepresented: %d/%d", heavyCount, n)
+	}
+}
+
+func TestRandomWeighted_ZeroWeight(t *testing.T) {
+	rw := NewRandomWeighted[int]()
+	rw.Add(1, 0)
+	rw.Add(2, 0)
+	// sum=0, should return zero value
+	if v := rw.Next(); v != 0 {
+		t.Fatalf("expected zero value with sum=0, got %d", v)
+	}
+}
+
+func TestRandomWeighted_Reset(t *testing.T) {
+	rw := NewRandomWeighted[int]()
+	rw.Add(1, 10)
+	rw.Add(2, 20)
+
+	rw.Reset()
+	if v := rw.Next(); v != 0 {
+		t.Fatalf("expected zero value after reset, got %d", v)
+	}
+
+	// Can add again after reset
+	rw.Add(3, 1)
+	if v := rw.Next(); v != 3 {
+		t.Fatalf("expected 3 after re-add, got %d", v)
+	}
+}
+
+func TestRandomWeighted_MultipleWeights(t *testing.T) {
+	rw := NewRandomWeighted[int]()
+	rw.Add(0, 1)
+	rw.Add(1, 2)
+	rw.Add(2, 3)
+	// Total weight = 6
+
+	counts := make(map[int]int)
+	const n = 6000
+	for i := 0; i < n; i++ {
+		counts[rw.Next()]++
+	}
+
+	// Item 2 should appear roughly 2x more than item 0
+	ratio := float64(counts[2]) / float64(counts[0])
+	if ratio < 1.5 || ratio > 3.5 {
+		t.Fatalf("expected ratio ~2.0 for item2/item0, got %.2f (counts: %v)", ratio, counts)
+	}
+}
+
+func TestRandomWeighted_StringItems(t *testing.T) {
+	rw := NewRandomWeighted[string]()
+	rw.Add("a", 1)
+	rw.Add("b", 1)
+
+	v := rw.Next()
+	if v != "a" && v != "b" {
+		t.Fatalf("expected 'a' or 'b', got %q", v)
+	}
+}
+
+func TestRandomWeighted_StructItems(t *testing.T) {
+	type item struct{ Name string }
+	rw := NewRandomWeighted[item]()
+	rw.Add(item{Name: "x"}, 1)
+	rw.Add(item{Name: "y"}, 1)
+
+	v := rw.Next()
+	if v.Name != "x" && v.Name != "y" {
+		t.Fatalf("unexpected item: %+v", v)
+	}
+}