feat(go): step 2 — per-phase request breakdown via httptrace

Instruments HTTP requests with net/http/httptrace.ClientTrace to capture
timestamps for DNS, TCP connect, TLS handshake, TTFB, and body transfer.
TLS row is omitted automatically for plain http:// URLs. Output is an
aligned text table with a separator before the Total row.

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 — Per-phase breakdown (Go, Step 2)
+
+- Instrumented requests with `net/http/httptrace.ClientTrace`
+- DNS lookup, TCP connect, TLS handshake, Server/TTFB, Transfer, Total phases
+- TLS row omitted automatically for plain `http://` URLs
+- Aligned text output with separator before Total
+
+---
+
 ## 2026-07-01 00:08 — Simple total latency (Go, Step 1)
 
 - `probe.Measure` performs an HTTP GET and records wall-clock total time