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/custom-usage-examples.md

@@ -7,7 +7,11 @@ make build
 # binary is now at go/latprobe
 ```
 
-Estimated runtimes assume a typical home broadband connection.
+**Concurrency note:** Since Step 7, `latprobe` probes URLs in parallel by default
+(`min(numURLs, 8)` workers). Estimated runtimes below reflect this — they are
+roughly _slowest-single-URL × samples_ rather than the sum across all URLs.
+Use `-c 1` to restore serial execution for the most accurate per-phase numbers.
+
 The timeout flag is shortened to `--timeout 3s` wherever non-routable IPs are
 used, so each timed-out sample fails in 3 s instead of the default 10 s.
 
@@ -32,7 +36,7 @@ used, so each timed-out sample fails in 3 s instead of the default 10 s.
   https://www.timeslive.co.za
 ```
 
-*Estimated runtime: ~30 s — shows per-phase breakdown once per site.*
+*Estimated runtime: ~5–8 s — all 10 sites measured in parallel; shows per-phase breakdown once per site.*
 
 ---
 
@@ -55,7 +59,7 @@ Same 10 sites, 10 requests each — reveals real latency distribution (min/avg/m
   https://www.timeslive.co.za
 ```
 
-*Estimated runtime: ~3–5 min — best for spotting jitter and TTFB variance.*
+*Estimated runtime: ~20–30 s — 8 workers run in parallel; wall time ≈ slowest single URL × 10 samples.*
 
 ---
 
@@ -86,7 +90,7 @@ Same as Example 2, machine-readable. Pipe into `jq` to extract specific phases.
   | jq '.[] | {url, avg_total_ms: .phases.total.avg_ms}'
 ```
 
-*Estimated runtime: ~3–5 min.*
+*Estimated runtime: ~20–30 s.*
 
 ---
 
@@ -110,7 +114,7 @@ The `.invalid` TLD is guaranteed NXDOMAIN by RFC 6761.
   https://nonexistent-host-two.invalid
 ```
 
-*Estimated runtime: ~2–3 min — DNS failures resolve near-instantly.*
+*Estimated runtime: ~20–30 s — DNS failures resolve near-instantly; 8 workers run in parallel.*
 
 ---
 
@@ -133,7 +137,7 @@ The `.invalid` TLD is guaranteed NXDOMAIN by RFC 6761.
   http://127.0.0.1:19999
 ```
 
-*Estimated runtime: ~2–3 min — refused connections fail immediately.*
+*Estimated runtime: ~20–30 s — refused connections fail immediately; 8 workers run in parallel.*
 
 ---
 
@@ -158,7 +162,7 @@ sample to 3 s instead of the default 10 s.
   https://203.0.113.1
 ```
 
-*Estimated runtime: ~3–4 min — timeout IPs add 3 s × 10 samples = 30 s each.*
+*Estimated runtime: ~35–40 s — all 10 URLs measured in parallel; the 2 timeout IPs each add 3 s × 10 samples = 30 s and are the bottleneck.*
 
 ---
 
@@ -182,7 +186,7 @@ would exit 0; with it, exit code becomes 6.
   https://httpbin.org/status/503
 ```
 
-*Estimated runtime: ~2–3 min — full timing captured even for error responses.*
+*Estimated runtime: ~20–30 s — full timing captured even for error responses; all 10 URLs measured in parallel.*
 
 ---
 
@@ -206,7 +210,7 @@ would exit 0; with it, exit code becomes 6.
   https://expired.badssl.com
 ```
 
-*Estimated runtime: ~2–3 min — TLS failures surface DNS + connect timing.*
+*Estimated runtime: ~20–30 s — TLS failures surface DNS + connect timing; all 10 URLs measured in parallel.*
 
 ---
 
@@ -237,7 +241,7 @@ HTTP error with `--fail` (exit 6). Highest code wins → **exit 5** (TLS is
   https://self-signed.badssl.com
 ```
 
-*Estimated runtime: ~5–7 min — the comprehensive stress test.*
+*Estimated runtime: ~35–45 s — 15 URLs measured with 8 workers; the 2 timeout IPs (30 s × each) are the bottleneck.*
 
 ---
 
@@ -276,7 +280,7 @@ JSON output lets you pipe results to `jq` for filtering.
   | jq '.[] | select(.failed > 0 or .status >= 400)'
 ```
 
-*Estimated runtime: ~30–60 s — fastest end-to-end check of all error paths.*
+*Estimated runtime: ~4–6 s — 15 URLs probed in parallel with `-n 1`; only the 2 s timeout IPs add meaningful delay.*
 
 ---