feat(go): step 3 — multiple URLs and -n sampling with min/avg/max

Add probe.Summarize to aggregate N Results into per-phase min/avg/max
stats. Wire up -n/--count flag in main to repeat each URL N times and
display an aligned three-column table. Single-sample output (n=1) is
unchanged. URLs are grouped with a blank line between them.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

go/main.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"strings"
+	"time"
 
 	"latprobe/internal/probe"
 )
@@ -39,22 +40,34 @@ func main() {
 		os.Exit(1)
 	}
 
-	// --json and -n are parsed but not yet functional (wired in Steps 3–4).
+	// --json parsed but not yet functional (wired in Step 4).
 	_ = jsonOut
-	_ = count
 
 	failed := false
 	for i, url := range urls {
 		if i > 0 {
 			fmt.Println()
 		}
-		r := probe.Measure(url)
-		if r.Err != nil {
-			fmt.Fprintf(os.Stderr, "error %s: %v\n", r.URL, r.Err)
-			failed = true
+
+		results := make([]probe.Result, 0, *count)
+		for j := 0; j < *count; j++ {
+			r := probe.Measure(url)
+			if r.Err != nil {
+				fmt.Fprintf(os.Stderr, "error %s (sample %d/%d): %v\n", url, j+1, *count, r.Err)
+				failed = true
+				break
+			}
+			results = append(results, r)
+		}
+		if len(results) == 0 {
 			continue
 		}
-		printResult(r)
+
+		if *count == 1 {
+			printResult(results[0])
+		} else {
+			printAggregate(probe.Summarize(results))
+		}
 	}
 
 	if failed {
@@ -62,6 +75,8 @@ func main() {
 	}
 }
 
+// ── single-sample output ──────────────────────────────────────────────────────
+
 func printResult(r probe.Result) {
 	fmt.Printf("%s  (%d)\n", r.URL, r.StatusCode)
 
@@ -75,17 +90,42 @@ func printResult(r probe.Result) {
 		{"Server (TTFB) ", r.TTFB},
 		{"Transfer      ", r.Transfer},
 	}
-
 	for _, ph := range phases {
 		if ph.p.Present {
-			fmt.Printf("  %s :  %8.2f ms\n", ph.label, msec(ph.p))
+			fmt.Printf("  %s :  %8.2f ms\n", ph.label, ms(ph.p.Duration))
 		}
 	}
-
 	fmt.Println("  " + strings.Repeat("─", 29))
-	fmt.Printf("  %s :  %8.2f ms\n", "Total         ", msec(r.Total))
+	fmt.Printf("  %s :  %8.2f ms\n", "Total         ", ms(r.Total.Duration))
 }
 
-func msec(p probe.Phase) float64 {
-	return float64(p.Duration.Microseconds()) / 1000
+// ── multi-sample output ───────────────────────────────────────────────────────
+
+func printAggregate(a probe.Aggregate) {
+	fmt.Printf("%s  (%d, %d samples)\n", a.URL, a.StatusCode, a.Count)
+	fmt.Printf("  %-14s    %9s  %9s  %9s\n", "", "min", "avg", "max")
+
+	phases := []struct {
+		label string
+		p     probe.PhaseStats
+	}{
+		{"DNS lookup    ", a.DNS},
+		{"TCP connect   ", a.Connect},
+		{"TLS handshake ", a.TLS},
+		{"Server (TTFB) ", a.TTFB},
+		{"Transfer      ", a.Transfer},
+	}
+	for _, ph := range phases {
+		if ph.p.Present {
+			fmt.Printf("  %s :  %6.2f ms  %6.2f ms  %6.2f ms\n",
+				ph.label, ms(ph.p.Min), ms(ph.p.Avg), ms(ph.p.Max))
+		}
+	}
+	fmt.Println("  " + strings.Repeat("─", 49))
+	fmt.Printf("  %s :  %6.2f ms  %6.2f ms  %6.2f ms\n",
+		"Total         ", ms(a.Total.Min), ms(a.Total.Avg), ms(a.Total.Max))
+}
+
+func ms(d time.Duration) float64 {
+	return float64(d.Microseconds()) / 1000
 }