feat: DuckDB quantiles in-process (gcc CGO), not Ladybug. (#34)

OQ3: internal/duckstats + bin/qa/stats.go. Zig stays Ladybug-only.
mikefarah/yq for small structured slices. Gitea #30.

internal/duckstats/duckstats_test.go

@@ -0,0 +1,51 @@
+package duckstats
+
+import (
+	"os"
+	"testing"
+)
+
+func TestQuantilesEmpty(t *testing.T) {
+	_, err := Quantiles(nil)
+	if err == nil {
+		t.Fatal("empty slice must error")
+	}
+}
+
+func TestQuantilesOdd(t *testing.T) {
+	s, err := Quantiles([]float64{1, 2, 3, 4, 5})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if s.N != 5 {
+		t.Fatalf("n=%d", s.N)
+	}
+	if s.Min != 1 || s.Max != 5 {
+		t.Fatalf("min=%v max=%v", s.Min, s.Max)
+	}
+	if s.P50 != 3 {
+		t.Fatalf("p50=%v want 3", s.P50)
+	}
+	if s.Avg != 3 {
+		t.Fatalf("avg=%v want 3", s.Avg)
+	}
+	if s.P95 < 4.5 || s.P95 > 5 {
+		t.Fatalf("p95=%v want in [4.5,5]", s.P95)
+	}
+}
+
+func TestCountJSONL(t *testing.T) {
+	dir := t.TempDir()
+	p := dir + "/rows.jsonl"
+	body := "{\"ms\":1}\n{\"ms\":2}\n{\"ms\":3}\n"
+	if err := os.WriteFile(p, []byte(body), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	n, err := CountJSONL(p)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 3 {
+		t.Fatalf("count=%d want 3", n)
+	}
+}