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>

CHANGELOG.md

@@ -4,6 +4,15 @@ All completed features are logged here in reverse-chronological order.
 
 ---
 
+## 2026-07-01 00:08 — Multiple URLs + sampling (Go, Step 3)
+
+- `-n`/`--count` flag repeats each URL N times and reports min/avg/max per phase
+- `probe.Summarize` aggregates a slice of Results into per-phase PhaseStats
+- Multi-sample output shows an aligned min/avg/max table with a header row
+- Single-sample output (n=1) is unchanged from Step 2
+
+---
+
 ## 2026-07-01 00:08 — Per-phase breakdown (Go, Step 2)
 
 - Instrumented requests with `net/http/httptrace.ClientTrace`