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>

docs/usage/step-2-phase-breakdown.md

@@ -0,0 +1,80 @@
+# Step 2 — Per-Phase Breakdown
+
+## What this step delivers
+
+`latprobe` now shows a full breakdown of every phase of an HTTP request:
+
+| Phase | What it measures |
+|-------|-----------------|
+| DNS lookup | Time to resolve the hostname (`DNSStart` → `DNSDone`) |
+| TCP connect | Time to establish the TCP connection (`ConnectStart` → `ConnectDone`) |
+| TLS handshake | Time to complete TLS negotiation — HTTPS only (`TLSHandshakeStart` → `TLSHandshakeDone`) |
+| Server (TTFB) | Time from request sent to first response byte (`WroteRequest` → `GotFirstResponseByte`) |
+| Transfer | Time to download the response body (`GotFirstResponseByte` → body closed) |
+| Total | Wall-clock time for the entire request |
+
+The TLS row is omitted automatically for plain `http://` URLs.
+
+Implemented via `net/http/httptrace.ClientTrace` — stdlib only, no external dependencies.
+
+## Build
+
+```sh
+cd go
+go build -o latprobe .
+```
+
+## Usage
+
+```sh
+latprobe <url> [url ...]
+```
+
+## Examples
+
+### HTTPS (all phases present)
+
+```sh
+$ ./latprobe https://example.com
+https://example.com  (200)
+  DNS lookup     :     18.21 ms
+  TCP connect    :     10.12 ms
+  TLS handshake  :     36.11 ms
+  Server (TTFB)  :     21.95 ms
+  Transfer       :      0.18 ms
+  ─────────────────────────────
+  Total          :     88.00 ms
+```
+
+### HTTP (TLS row omitted)
+
+```sh
+$ ./latprobe http://example.com
+http://example.com  (200)
+  DNS lookup     :      1.64 ms
+  TCP connect    :      9.29 ms
+  Server (TTFB)  :     17.96 ms
+  Transfer       :      0.07 ms
+  ─────────────────────────────
+  Total          :     29.56 ms
+```
+
+### Multiple URLs
+
+```sh
+$ ./latprobe https://example.com https://www.google.com
+https://example.com  (200)
+  DNS lookup     :     18.21 ms
+  ...
+
+https://www.google.com  (200)
+  DNS lookup     :      7.15 ms
+  ...
+```
+
+## Notes
+
+- **DNS absent on repeat connections**: when the OS has cached the DNS result,
+  the DNS phase may be very short or absent. This is expected.
+- **Phases don't always sum exactly to Total**: the trace hooks introduce
+  negligible overhead between events; the difference is typically < 1 ms.