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
 

docs/usage/step-7-concurrency.md

@@ -0,0 +1,76 @@
+# Step 7 — Concurrency
+
+`latprobe` now probes multiple URLs in parallel, cutting wall-clock time for
+large runs without sacrificing measurement accuracy.
+
+## How it works
+
+| Axis | Behaviour |
+|------|-----------|
+| Across URLs | **Parallel** — up to `-c` workers run simultaneously |
+| Samples of one URL (`-n`) | **Sequential** — always serial within a URL |
+
+Keeping samples sequential ensures that min/avg/max statistics for a single URL
+reflect genuine latency variability, not artificial load created by firing
+multiple requests at the same server at once.
+
+Output is always printed in **input order**, regardless of which URL finishes
+first.
+
+## Flag
+
+```
+-c, --concurrency int   Max URLs probed in parallel (0 = auto, default)
+```
+
+| Value | Behaviour |
+|-------|-----------|
+| `0` (default) | Auto: `min(numURLs, 8)` — scales with workload, caps at 8 |
+| `1` | Fully serial — identical to the old behaviour; best for precision |
+| `N > 1` | Explicit worker cap |
+
+## When to use serial mode
+
+For the most accurate latency numbers — especially when comparing sites —
+use `-c 1`. Concurrent probes share your local NIC, DNS resolver, and CPU,
+which can inflate timings on slower machines or fast batch runs.
+
+```sh
+# High fidelity: one URL at a time
+./go/latprobe -c 1 -n 10 https://www.google.com https://www.bbc.co.uk
+
+# Speed: all four probed in parallel (auto default would do this anyway)
+./go/latprobe -c 4 -n 10 https://www.google.com https://www.bbc.co.uk \
+  https://www.lemonde.fr https://www.abc.net.au
+```
+
+## Example: speedup in practice
+
+Five sites, 5 samples each — sequential vs parallel:
+
+```sh
+# Serial (-c 1): ~wall time ≈ sum of all RTTs × 5
+time ./go/latprobe -c 1 -n 5 \
+  https://www.google.com https://www.bbc.co.uk \
+  https://www.lemonde.fr https://www.spiegel.de https://www.abc.net.au
+# real ~1m15s  (5 sites × ~15s sequential)
+
+# Parallel (default): ~wall time ≈ slowest single URL × 5
+time ./go/latprobe -n 5 \
+  https://www.google.com https://www.bbc.co.uk \
+  https://www.lemonde.fr https://www.spiegel.de https://www.abc.net.au
+# real ~18s  (all 5 measured at once, gated by the slowest)
+```
+
+## Exit codes
+
+Exit codes work exactly as before: when multiple failure types occur the
+**highest** code wins. Concurrency does not change this — exit codes are
+accumulated after all workers complete.
+
+## Race safety
+
+The implementation uses a semaphore channel (`chan struct{}`) and
+`sync.WaitGroup` with each goroutine writing only its own indexed result slot,
+so there is no shared mutable state. `http.DefaultClient` is safe for concurrent
+use. Verified clean with `go test -race` (`make go-test-race`).