feat(go): step 1 — total wall-clock latency per URL

probe.Measure performs an HTTP GET and records total elapsed time from
request start to body fully read. main.go prints URL, status code, and
total for each positional URL argument, exiting 1 on any failure.

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

README.md

@@ -90,7 +90,7 @@ https://example.com  (5 samples)
 | Step | Feature | Status |
 |------|---------|--------|
 | 0 | Project scaffold — directory structure, `go.mod`, minimal binary | ✅ Done |
-| 1 | Simple total latency — single URL, wall-clock time | ⬜ Pending |
+| 1 | Simple total latency — single URL, wall-clock time | ✅ Done |
 | 2 | Per-phase breakdown — DNS, TCP, TLS, TTFB, transfer (`net/http/httptrace`) | ⬜ Pending |
 | 3 | Multiple URLs + `--count`/`-n` — min/avg/max aggregates | ⬜ Pending |
 | 4 | `--json` output flag | ⬜ Pending |