Add script/loadtest to measure throughput, latency and memory (closes #81)

script/loadtest [duration [clients]] (make loadtest, defaults 10s and 4)
measures pixad in three scenarios: hit (one cached image), miss (a new
source image every request) and herd (each new source image asked for
by all clients at once). For each it prints vegeta's report (requests
per second, p50/p95/p99 latency, status codes), pixad's peak resident
memory and how many requests reached the origin. It is a benchmark, not
run by script/check. The origin it uses, internal/loadtestorigin behind
cmd/loadtest-origin, answers every path with one generated JPEG. Bad
arguments are refused before the build. README.md says how to run and
read it and keeps 1-5k r/s as a target not yet measured at scale.

Model: opus-5-5

internal/loadtestorigin/loadtestorigin_internal_test.go

@@ -0,0 +1,51 @@
+package loadtestorigin
+
+import (
+	"bytes"
+	"image/jpeg"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestEveryPathServesTheSameJPEG checks that the origin answers any path with
+// 200 and the same JPEG, so every new path script/loadtest asks pixad for is
+// a valid source image.
+func TestEveryPathServesTheSameJPEG(t *testing.T) {
+	t.Parallel()
+
+	photo, err := makeJPEG()
+	if err != nil {
+		t.Fatalf("makeJPEG: %v", err)
+	}
+
+	size, err := jpeg.DecodeConfig(bytes.NewReader(photo))
+	if err != nil {
+		t.Fatalf("the image does not decode as a JPEG: %v", err)
+	}
+
+	if size.Width != imageWidth || size.Height != imageHeight {
+		t.Errorf("the image is %dx%d, want %dx%d",
+			size.Width, size.Height, imageWidth, imageHeight)
+	}
+
+	handler := newHandler(photo)
+
+	for _, path := range []string{"/", "/miss/1.jpg", "/herd/2.jpg"} {
+		rec := httptest.NewRecorder()
+		handler.ServeHTTP(rec, httptest.NewRequestWithContext(
+			t.Context(), http.MethodGet, path, nil))
+
+		if rec.Code != http.StatusOK {
+			t.Errorf("%s: status = %d, want %d", path, rec.Code, http.StatusOK)
+		}
+
+		if ct := rec.Header().Get("Content-Type"); ct != "image/jpeg" {
+			t.Errorf("%s: Content-Type = %q, want image/jpeg", path, ct)
+		}
+
+		if !bytes.Equal(rec.Body.Bytes(), photo) {
+			t.Errorf("%s: the body is not the image", path)
+		}
+	}
+}