feat(go): step 5 — failure handling with classified exit codes

Classify network failures (dns/connect/timeout/tls) with distinct exit
codes 2-5. Add --timeout (default 10s) via context.WithTimeout. Add
--fail for exit 6 on HTTP status >= 400. Preserve partial phase timing
up to the failure point. -n sampling continues on network failures,
aggregating successes and reporting fail counts. JSON extended with
succeeded/failed/errors fields.

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

README.md

@@ -94,6 +94,7 @@ https://example.com  (5 samples)
 | 2 | Per-phase breakdown — DNS, TCP, TLS, TTFB, transfer (`net/http/httptrace`) | ✅ Done |
 | 3 | Multiple URLs + `--count`/`-n` — min/avg/max aggregates | ✅ Done |
 | 4 | `--json` output flag | ✅ Done |
+| 5 | Failure handling — `--timeout`, `--fail`, distinct exit codes, partial timing | ✅ Done |
 
 ### Python