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.go

@@ -0,0 +1,47 @@
+// Package duckstats runs in-process DuckDB for columnar aggregates.
+// Graph facts stay in Ladybug. Web-search KV cache stays modernc sqlite.
+package duckstats
+
+import (
+	"database/sql"
+	"fmt"
+
+	_ "github.com/duckdb/duckdb-go/v2"
+)
+
+type Stats struct {
+	N   int     `json:"n"`
+	Min float64 `json:"min"`
+	P50 float64 `json:"p50"`
+	P95 float64 `json:"p95"`
+	Max float64 `json:"max"`
+	Avg float64 `json:"avg"`
+}
+
+func Quantiles(samples []float64) (Stats, error) {
+	if len(samples) == 0 {
+		return Stats{}, fmt.Errorf("duckstats: empty samples")
+	}
+	db, err := sql.Open("duckdb", "")
+	if err != nil {
+		return Stats{}, err
+	}
+	defer db.Close()
+	var s Stats
+	err = db.QueryRow(`
+SELECT count(v), min(v), quantile_cont(v, 0.5), quantile_cont(v, 0.95), max(v), avg(v)
+FROM (SELECT unnest(?) AS v)`, samples).Scan(
+		&s.N, &s.Min, &s.P50, &s.P95, &s.Max, &s.Avg)
+	return s, err
+}
+
+func CountJSONL(path string) (int64, error) {
+	db, err := sql.Open("duckdb", "")
+	if err != nil {
+		return 0, err
+	}
+	defer db.Close()
+	var n int64
+	err = db.QueryRow(`SELECT count(*) FROM read_json_auto(?)`, path).Scan(&n)
+	return n, err
+}