Resize an example under the rig, since a resize is its own case

`--resize WxH@Hz` changes the output once the app is up and screenshots
after, so "it lands where a cold start at that size does" is a command
rather than a procedure. That check caught both of #16's defects and nothing
in `cargo test` can see it; it now passes byte for byte on `tabs` and `text`
for this branch.

Run one at a time: the rig reuses a single compositor and a single output,
so two invocations at once resize each other's window and quietly screenshot
the wrong thing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/run-headless.sh

@@ -17,6 +17,10 @@
 # custom one would otherwise inherit the other's output and quietly screenshot
 # the wrong size.
 #
+# `--resize WxH@Hz` changes the output under the app once it is up, then
+# screenshots. A resize is its own case: what it has to match is a cold start
+# at that size, byte for byte, and nothing in `cargo test` can see it.
+#
 # `--replay FILE` drives a `.touch` recording into the window through
 # `replay-touch`, which reads it with the same parser `iris::harness` uses. A
 # recording is `<ms> down|move|up <x> <y>` in the output's own pixels. With
@@ -46,6 +50,7 @@ run="${XDG_RUNTIME_DIR:-/tmp}/iris-headless"
 seconds=3
 shot=""
 replay=""
+resize=""
 example=""
 kind=example
 mode=1920x1200@60Hz
@@ -57,13 +62,14 @@ while [ $# -gt 0 ]; do
         --seconds) seconds=$2; shift 2 ;;
         --bin) kind=bin; shift ;;
         --mode) mode=$2; shift 2 ;;
+        --resize) resize=$2; shift 2 ;;
         --replay) replay=$2; shift 2 ;;
         --dir) workdir=$(cd "$2" && pwd); shift 2 ;;
         --) shift; break ;;
         *) example=$1; shift ;;
     esac
 done
-[ -n "$example" ] || { echo "usage: $0 NAME [--bin] [--dir DIR] [--mode WxH@Hz] [--replay TOUCH] [--shot PNG] [--seconds N] [-- cargo args]" >&2; exit 2; }
+[ -n "$example" ] || { echo "usage: $0 NAME [--bin] [--dir DIR] [--mode WxH@Hz] [--resize WxH@Hz] [--replay TOUCH] [--shot PNG] [--seconds N] [-- cargo args]" >&2; exit 2; }
 [ -z "$replay" ] || [ -f "$replay" ] || { echo "run-headless: no touch script at $replay" >&2; exit 2; }
 [ -z "$shot" ] || need grim "the screenshot --shot writes"
 
@@ -142,6 +148,12 @@ while [ $i -lt "$((seconds * 2))" ]; do
     i=$((i + 1)); sleep 0.5
 done
 
+if [ -n "$resize" ] && kill -0 "$pid" 2>/dev/null; then
+    swaymsg output HEADLESS-1 mode "$resize" >/dev/null
+    echo "run-headless: resized to $resize" >&2
+    sleep 2
+fi
+
 if [ -n "$replay" ] && kill -0 "$pid" 2>/dev/null; then
     if [ -n "$shot" ]; then
         grim "${shot%.png}-before.png"