Add URL-level concurrency (-c flag, Step 7)

Probe multiple URLs in parallel with a bounded worker pool; the N samples
of each URL remain sequential to preserve accurate min/avg/max statistics.
Default auto-concurrency is min(numURLs, 8); -c 1 restores serial mode.
Output is always buffered and printed in original input order. Verified
clean with go test -race.

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

CHANGELOG.md

@@ -4,6 +4,24 @@ All completed features are logged here in reverse-chronological order.
 
 ---
 
+## 2026-07-01 01:23 — Concurrency (Go, Step 7)
+
+- Added `-c`/`--concurrency` flag: max URLs probed in parallel (0 = auto)
+- Default auto-concurrency: `min(numURLs, 8)` — scales with workload, caps at 8
+- Parallelism is **across URLs only**; the N samples of each URL stay sequential
+  to preserve the accuracy of per-URL min/avg/max statistics
+- Output buffered and printed in original input order (deterministic for both
+  text and JSON), exit-code accumulation unchanged
+- Implementation: semaphore channel + `sync.WaitGroup`, each goroutine writes
+  only its own indexed result slot — verified clean with `go test -race`
+- Added `make go-test-race` target to the Makefile
+- 3 new tests: `TestRunConcurrentOrder`, `TestConcurrentWorstCode`,
+  `TestConcurrentJSONOrder`
+- Updated runtime estimates in `docs/custom-usage-examples.md`
+- User doc: `docs/usage/step-7-concurrency.md`
+
+---
+
 ## 2026-07-01 01:05 — Root Makefile
 
 - Namespaced targets: `go-build`, `go-run`, `go-test`, `go-test-verbose`, `go-check`, `go-cover`, `go-fmt`, `go-vet`, `go-tidy`, `go-install`, `go-lint`, `go-clean`