feat(go): step 4 — --json output flag

Add --json flag that emits all results as a JSON array. Schema always
uses min/avg/max per phase (consistent regardless of -n); phases absent
from the request are omitted from the phases object. Output is buffered
until all URLs complete, then printed as one pretty-printed array.

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

docs/usage/step-4-json-output.md

@@ -0,0 +1,105 @@
+# Step 4 — JSON Output
+
+## What this step delivers
+
+A `--json` flag that emits all results as a JSON array instead of the text
+table. The text format remains the default.
+
+The JSON schema is always the aggregate shape (min/avg/max per phase), even
+for a single sample — so the schema is stable regardless of `-n`.
+
+Phases absent from the request (e.g. TLS for `http://`) are omitted from the
+`phases` object.
+
+## Build
+
+```sh
+cd go
+go build -o latprobe .
+```
+
+## Usage
+
+```sh
+latprobe --json [other flags] <url> [url ...]
+```
+
+## JSON Schema
+
+```json
+[
+  {
+    "url":     "https://example.com",
+    "status":  200,
+    "samples": 1,
+    "phases": {
+      "dns":      { "min_ms": 17.17, "avg_ms": 17.17, "max_ms": 17.17 },
+      "connect":  { "min_ms": 12.01, "avg_ms": 12.01, "max_ms": 12.01 },
+      "tls":      { "min_ms": 22.78, "avg_ms": 22.78, "max_ms": 22.78 },
+      "ttfb":     { "min_ms": 12.99, "avg_ms": 12.99, "max_ms": 12.99 },
+      "transfer": { "min_ms":  0.18, "avg_ms":  0.18, "max_ms":  0.18 },
+      "total":    { "min_ms": 66.19, "avg_ms": 66.19, "max_ms": 66.19 }
+    }
+  }
+]
+```
+
+Phase keys: `dns`, `connect`, `tls` (HTTPS only), `ttfb`, `transfer`, `total`.
+All `*_ms` values are floating-point milliseconds.
+
+## Examples
+
+### Single URL, single sample
+
+```sh
+$ ./latprobe --json https://example.com
+[
+  {
+    "url": "https://example.com",
+    "status": 200,
+    "samples": 1,
+    "phases": { ... }
+  }
+]
+```
+
+### Multiple samples — min/avg/max diverge
+
+```sh
+$ ./latprobe --json -n 5 https://example.com
+[
+  {
+    "url": "https://example.com",
+    "status": 200,
+    "samples": 5,
+    "phases": {
+      "total": { "min_ms": 45.00, "avg_ms": 52.30, "max_ms": 61.80 },
+      ...
+    }
+  }
+]
+```
+
+### Pipe to jq
+
+```sh
+# Extract avg total latency for each URL
+$ ./latprobe --json -n 3 https://example.com https://www.google.com \
+  | jq '.[] | {url, avg_total_ms: .phases.total.avg_ms}'
+
+{
+  "url": "https://example.com",
+  "avg_total_ms": 52.3
+}
+{
+  "url": "https://www.google.com",
+  "avg_total_ms": 82.8
+}
+```
+
+## Notes
+
+- Output is buffered until all URLs are measured, then printed as one array.
+  Errors for individual URLs are written to stderr; that URL is excluded from
+  the JSON array, and the process exits with code 1.
+- The text format is unchanged and remains the default (no `--json` flag).