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.

bin/qa/stats.go

@@ -0,0 +1,73 @@
+//usr/bin/env go run -tags=qa_stats "$0" "$@"; exit
+//go:build qa_stats
+//
+// bin/qa/stats.go - DuckDB quantiles over a JSON number array or JSONL count.
+//
+//	./bin/qa/stats.go <<< '[1,2,3,4,5]'
+//	./bin/qa/stats.go --jsonl rows.jsonl
+//
+// NOTE: never run `gofmt -w` on this file — it breaks the shebang.
+// DuckDB CGO needs gcc/g++ (not Zig). After eval "$(bin/cgo/zig env)":
+//   CC=gcc CXX=g++ CGO_CFLAGS= CGO_LDFLAGS= ./bin/qa/stats.go
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"github.com/eSlider/2dph/internal/duckstats"
+)
+
+func main() {
+	os.Exit(run(os.Args[1:]))
+}
+
+func run(args []string) int {
+	jsonl := ""
+	for i := 0; i < len(args); i++ {
+		a := args[i]
+		switch {
+		case a == "--jsonl" && i+1 < len(args):
+			i++
+			jsonl = args[i]
+		case strings.HasPrefix(a, "--jsonl="):
+			jsonl = strings.TrimPrefix(a, "--jsonl=")
+		case a == "-h" || a == "--help":
+			fmt.Fprintln(os.Stderr, "bin/qa/stats.go [--jsonl FILE]  # stdin = JSON [float,…]")
+			return 0
+		default:
+			fmt.Fprintln(os.Stderr, "unknown arg:", a)
+			return 2
+		}
+	}
+	if jsonl != "" {
+		n, err := duckstats.CountJSONL(jsonl)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			return 1
+		}
+		fmt.Printf("n: %d\n", n)
+		return 0
+	}
+	raw, err := io.ReadAll(os.Stdin)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		return 1
+	}
+	var samples []float64
+	if err := json.Unmarshal(raw, &samples); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		return 1
+	}
+	s, err := duckstats.Quantiles(samples)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		return 1
+	}
+	fmt.Printf("n: %d\nmin: %g\np50: %g\np95: %g\nmax: %g\navg: %g\n",
+		s.N, s.Min, s.P50, s.P95, s.Max, s.Avg)
+	return 0
+}