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

script/loadtest [duration [clients]], or make loadtest, is a benchmark
that script/check does not run. It builds the image and vegeta from a
pinned commit, then for each scenario starts a new pixad container and
a new cmd/loadtest-origin container, an upstream host that answers every
path with one generated JPEG: a cached image (hit), a new source image
per request (miss), and each new source image asked for by all clients
at once (herd). It prints vegeta's report, pixad's peak resident memory
and the requests the origin got. The containers share a network in
203.0.113.0/24, as pixad refuses private and local upstream addresses.
README.md says how to run and read it; TODO.md records a small baseline.

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 .