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

Makefile

@@ -1,4 +1,4 @@
-.PHONY: bootstrap setup check lint test fmt fmt-check build clean docker docker-smoke docker-versioned docker-test devserver devserver-stop hooks
+.PHONY: bootstrap setup check lint test fmt fmt-check build clean docker docker-smoke docker-versioned docker-test devserver devserver-stop hooks loadtest
 
 VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
 LDFLAGS := -X main.Version=$(VERSION)
@@ -59,6 +59,11 @@ docker:
 docker-smoke:
 	@script/docker-smoke
 
+# Measure throughput, latency and peak memory with the default duration and
+# number of clients (needs Docker and Go; a benchmark, not part of check)
+loadtest:
+	@script/loadtest
+
 # Build Docker image tagged pixad:$(VERSION) and pixad:latest
 docker-versioned:
 	docker build --build-arg VERSION=$(VERSION) -t pixad:$(VERSION) -t pixad:latest .