Check script/loadtest's duration before the build

A duration must be a whole number above zero followed by ms, s, m or
h; anything else prints the usage line and exits 2 before the image
build, as a bad number of clients already did. vegeta takes 0 to mean
no end. The load test's entry moves to the top of Completed Steps in
TODO.md.

Model: opus-5-5

script/loadtest

@@ -32,14 +32,27 @@ VEGETA_COMMIT=4b240c3089fa4aa10816542d64a74294d974211f
 # routed on the internet.
 SUBNET=203.0.113.0/24
 
+usage() {
+    echo "usage: script/loadtest [duration [clients]]" >&2
+    exit 2
+}
+
 main() {
     duration="${1:-10s}"
     clients="${2:-4}"
+    # The duration is a whole number, not zero (vegeta takes 0 to mean no
+    # end), followed by ms, s, m or h.
+    case "$duration" in
+        *ms) number="${duration%ms}" ;;
+        *s | *m | *h) number="${duration%?}" ;;
+        *) usage ;;
+    esac
+    case "$number" in
+        "" | *[!0-9]*) usage ;;
+    esac
+    [ "$number" -gt 0 ] || usage
     case "$clients" in
-        *[!0-9]* | 0)
-            echo "usage: script/loadtest [duration [clients]]" >&2
-            exit 2
-            ;;
+        *[!0-9]* | 0) usage ;;
     esac
 
     cd "$ROOT"