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

TODO.md

@@ -31,6 +31,21 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-04 load test (closes #81): `script/loadtest [duration [clients]]`
+  (`make loadtest`, defaults `10s` and `4`), a benchmark that `script/check`
+  does not run, measures three scenarios, each against a new pixad container and
+  a new upstream host, `cmd/loadtest-origin`: `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, latency percentiles, status codes), pixad's peak resident memory and
+  the requests that reached the origin. `README.md` says how to run it and read
+  it, and keeps 1-5k r/s as a target not yet measured. First measurement, with
+  the defaults on a shared 48-CPU machine with other work running: a baseline
+  for later changes, not a test of the target. `hit` 1413 r/s, p50 0.7 ms, p95
+  8.7 ms, p99 44 ms, peak 53 MiB (4 clients that each wait for their answer, so
+  not pixad's limit); `miss` 70 r/s, p50 52 ms, p95 91 ms, p99 122 ms, peak 100
+  MiB, one fetch per request; `herd` 74 r/s, p50 52 ms, p95 69 ms, p99 111 ms,
+  peak 60 MiB, 188 fetches for 749 requests.
 - 2026-10-04 the CI checkout fetches the tags (closes #208): the checkout step
   in `.gitea/workflows/check.yml` sets `fetch-depth: 0`, as `REPO_POLICIES.md`
   asks of a repo that takes its version from the tags, so a CI build of a tagged
@@ -652,4 +667,5 @@ P2: security: per-IP rate limiting on the image routes
     - optional Sentry error reporting
     - comprehensive request logging
     - Prometheus performance metrics
-    - load tests to verify the 1k to 5k req/s target
+    - measure the 1k to 5k req/s target with `script/loadtest` on a machine not
+      shared with other work