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>

docs/usage/makefile.md

@@ -32,6 +32,7 @@ make help         # list all targets with descriptions
 | `go-run` | Build and run (pass flags via `ARGS=`) |
 | `go-test` | `go test ./...` |
 | `go-test-verbose` | `go test -v ./...` — per-case PASS/FAIL output |
+| `go-test-race` | `go test -race ./...` — run tests with the data-race detector |
 | `go-check` | `go-fmt` + `go-vet` + `go-test` |
 | `go-cover` | Coverage report → `go/coverage.html` |
 | `go-fmt` | `gofmt -w go/` |
@@ -54,6 +55,9 @@ make test
 # Verbose output showing each test case
 make go-test-verbose
 
+# Run tests with the race detector (verify concurrency safety)
+make go-test-race
+
 # Pre-commit gate (format, vet, test all in one)
 make check