feat(hack): add scripts to drive the dev environment headless

Start QEMU in the background with a QMP socket and console logs, take
screenshots, press keys and wait for serial output, so the graphical boot
menu can be driven without a terminal or a window. boot.sh gets opt-in
knobs for this and behaves as before otherwise.

hack/dev/run/lib.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+# Shared by the other scripts in this directory: source it after changing to the repository root.
+# shellcheck disable=SC2034
+
+# common.sh reads the architecture from the first argument, which belongs to the calling script here.
+load_common() {
+	set --
+	. "hack/common.sh"
+}
+load_common
+
+RUN_DIR="$(pwd)/target/run/${TARGET_ARCH}"
+SHOT_DIR="${RUN_DIR}/shots"
+SERIAL_LOG="${RUN_DIR}/serial.log"
+CONSOLE_LOG="${RUN_DIR}/console.log"
+QEMU_LOG="${RUN_DIR}/qemu.log"
+PID_FILE="${RUN_DIR}/qemu.pid"
+
+# Unix socket paths are limited to about 104 bytes on macOS, which a deep worktree exceeds.
+QMP_SOCK="${TMPDIR:-/tmp}"
+QMP_SOCK="${QMP_SOCK%/}/sprout-${TARGET_ARCH}.qmp"
+
+die() {
+	echo "ERROR: ${*}" >/dev/stderr
+	exit 1
+}
+
+is_running() {
+	[ -f "${PID_FILE}" ] && kill -0 "$(cat "${PID_FILE}")" 2>/dev/null
+}
+
+require_running() {
+	is_running || die "no ${TARGET_ARCH} QEMU is running, start one with hack/dev/run/start.sh"
+}