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>

README.md

@@ -28,12 +28,16 @@ Both implementations produce identical CLI behaviour and output formats.
 latprobe [flags] <url> [url ...]
 
 Flags:
-  -n, --count int   Number of requests per URL (default 1)
-      --json        Output results as JSON instead of text
+  -n, --count int          Number of requests per URL (default 1)
+  -c, --concurrency int    Max URLs probed in parallel, 0 = auto (default min(numURLs,8))
+      --timeout duration   Request timeout, e.g. 10s, 500ms (default 10s)
+      --fail               Exit non-zero on HTTP status >= 400 (exit code 6)
+      --json               Output results as JSON instead of text
 
 Examples:
   latprobe https://example.com
   latprobe -n 5 https://example.com https://www.google.com
+  latprobe -c 1 -n 10 https://example.com   # serial, most accurate
   latprobe --json https://example.com | jq .
 ```
 
@@ -96,6 +100,7 @@ https://example.com  (5 samples)
 | 4 | `--json` output flag | ✅ Done |
 | 5 | Failure handling — `--timeout`, `--fail`, distinct exit codes, partial timing | ✅ Done |
 | 6 | Integration tests — in-process matrix + subprocess smoke tests | ✅ Done |
+| 7 | Concurrency — `-c` worker pool across URLs; samples stay serial per URL | ✅ Done |
 
 ### Python
 
@@ -116,9 +121,10 @@ make test         # run all tests
 make check        # fmt + vet + test (pre-commit gate)
 make go-run ARGS="https://example.com"
 make go-test-verbose
-make go-cover     # coverage report → go/coverage.html
-make go-lint      # golangci-lint
-make clean        # remove build artifacts
+make go-cover       # coverage report → go/coverage.html
+make go-test-race   # run tests with race detector
+make go-lint        # golangci-lint
+make clean          # remove build artifacts
 ```
 
 See [`docs/usage/makefile.md`](docs/usage/makefile.md) for the full target reference.