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>

docs/usage/step-1-total-latency.md

@@ -0,0 +1,57 @@
+# Step 1 — Simple Total Latency
+
+## What this step delivers
+
+`latprobe` accepts one or more URLs and prints the total wall-clock time of
+each HTTP GET request (including body download). No per-phase breakdown yet —
+that comes in Step 2.
+
+## Build
+
+```sh
+cd go
+go build -o latprobe .
+```
+
+## Usage
+
+```sh
+latprobe <url> [url ...]
+```
+
+## Examples
+
+Single URL:
+
+```sh
+$ ./latprobe https://example.com
+https://example.com                            200  70ms
+```
+
+Multiple URLs:
+
+```sh
+$ ./latprobe https://example.com https://www.google.com
+https://example.com                            200  70ms
+https://www.google.com                         200  127ms
+```
+
+Error (unreachable host):
+
+```sh
+$ ./latprobe https://doesnotexist.invalid
+error https://doesnotexist.invalid: Get "https://doesnotexist.invalid": ...
+# exits with code 1
+```
+
+## Output columns
+
+| Column | Description |
+|--------|-------------|
+| URL | The requested URL |
+| Status | HTTP response status code |
+| Total | Wall-clock time from request start to body fully received |
+
+## Flags accepted (not yet functional)
+
+`-n`/`--count` and `--json` are parsed but produce no effect until Steps 3–4.