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>

docs/usage/step-3-sampling.md

@@ -0,0 +1,79 @@
+# Step 3 — Multiple URLs + Sampling
+
+## What this step delivers
+
+- Multiple URLs accepted as positional arguments; each is measured in turn.
+- `-n`/`--count` flag repeats each URL N times and reports **min / avg / max**
+  per phase instead of a single value.
+- With `n=1` (the default) the output is identical to Step 2.
+
+## Build
+
+```sh
+cd go
+go build -o latprobe .
+```
+
+## Usage
+
+```sh
+latprobe [flags] <url> [url ...]
+
+  -n, --count int   Number of requests per URL (default 1)
+```
+
+## Examples
+
+### Default (single sample, unchanged from Step 2)
+
+```sh
+$ ./latprobe https://example.com
+https://example.com  (200)
+  DNS lookup     :     13.74 ms
+  TCP connect    :     11.20 ms
+  TLS handshake  :     22.92 ms
+  Server (TTFB)  :     14.88 ms
+  Transfer       :      0.11 ms
+  ─────────────────────────────
+  Total          :     63.97 ms
+```
+
+### Multiple samples (`-n`)
+
+```sh
+$ ./latprobe -n 5 https://example.com
+https://example.com  (200, 5 samples)
+                          min        avg        max
+  DNS lookup     :    1.50 ms    1.60 ms    1.70 ms
+  TCP connect    :   10.20 ms   10.80 ms   11.50 ms
+  TLS handshake  :   21.00 ms   22.10 ms   23.40 ms
+  Server (TTFB)  :   12.00 ms   13.50 ms   15.20 ms
+  Transfer       :    0.07 ms    0.09 ms    0.12 ms
+  ─────────────────────────────────────────────────
+  Total          :   45.00 ms   48.10 ms   52.00 ms
+```
+
+### Multiple URLs
+
+```sh
+$ ./latprobe -n 3 https://example.com https://www.google.com
+https://example.com  (200, 3 samples)
+                          min        avg        max
+  ...
+  ─────────────────────────────────────────────────
+  Total          :   ...
+
+https://www.google.com  (200, 3 samples)
+                          min        avg        max
+  ...
+```
+
+## Notes
+
+- **DNS and connect on repeated requests**: after the first request the OS
+  DNS cache and HTTP keep-alive may eliminate those phases in subsequent
+  samples. The spread in TCP connect and DNS between min and max shows the
+  true variance when caches are cold.
+- **Partial failure**: if a sample fails mid-run, that URL is skipped and
+  the exit code is 1. Successful samples already collected are discarded for
+  that URL.