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/internal/probe/aggregate.go

@@ -0,0 +1,71 @@
+package probe
+
+import "time"
+
+// PhaseStats holds min/avg/max for a phase across N samples.
+type PhaseStats struct {
+	Min, Avg, Max time.Duration
+	Present       bool
+}
+
+// Aggregate holds per-phase statistics over a set of Results.
+type Aggregate struct {
+	URL        string
+	Count      int
+	StatusCode int
+	DNS        PhaseStats
+	Connect    PhaseStats
+	TLS        PhaseStats
+	TTFB       PhaseStats
+	Transfer   PhaseStats
+	Total      PhaseStats
+}
+
+// Summarize computes min/avg/max per phase from a slice of Results.
+func Summarize(results []Result) Aggregate {
+	a := Aggregate{Count: len(results)}
+	if len(results) == 0 {
+		return a
+	}
+	a.URL = results[0].URL
+	a.StatusCode = results[len(results)-1].StatusCode
+
+	a.DNS = phaseStats(results, func(r Result) Phase { return r.DNS })
+	a.Connect = phaseStats(results, func(r Result) Phase { return r.Connect })
+	a.TLS = phaseStats(results, func(r Result) Phase { return r.TLS })
+	a.TTFB = phaseStats(results, func(r Result) Phase { return r.TTFB })
+	a.Transfer = phaseStats(results, func(r Result) Phase { return r.Transfer })
+	a.Total = phaseStats(results, func(r Result) Phase { return r.Total })
+
+	return a
+}
+
+func phaseStats(results []Result, get func(Result) Phase) PhaseStats {
+	var durations []time.Duration
+	for _, r := range results {
+		if p := get(r); p.Present {
+			durations = append(durations, p.Duration)
+		}
+	}
+	if len(durations) == 0 {
+		return PhaseStats{}
+	}
+
+	min, max := durations[0], durations[0]
+	var sum time.Duration
+	for _, d := range durations {
+		if d < min {
+			min = d
+		}
+		if d > max {
+			max = d
+		}
+		sum += d
+	}
+	return PhaseStats{
+		Min:     min,
+		Avg:     sum / time.Duration(len(durations)),
+		Max:     max,
+		Present: true,
+	}
+}