build: kill previous watch instance when starting pnpm watch

Track the tsdown watcher pid in .tmp/watch.pid; a new watch kills the
previous instance first and the pid file is cleaned up on exit.

package.json

@@ -34,7 +34,7 @@
     "build": "tsdown",
     "lint": "oxlint",
     "lint:fix": "oxlint --fix",
-    "watch": "tsdown --watch",
+    "watch": "bash scripts/watch.sh",
     "register": "bash scripts/register.sh",
     "typecheck": "tsc --noEmit && tsc --noEmit -p tsconfig.tests.json",
     "test": "pnpm run typecheck && vitest run --coverage",

scripts/watch.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+# Starts tsdown --watch, killing the previous watch instance (pid tracked in .tmp/watch.pid) first.
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$ROOT"
+
+PID_FILE="$ROOT/.tmp/watch.pid"
+mkdir -p "$ROOT/.tmp"
+
+if [[ -f "$PID_FILE" ]]; then
+  old_pid="$(cat "$PID_FILE" 2>/dev/null || true)"
+  if [[ "$old_pid" =~ ^[0-9]+$ ]] && kill -0 "$old_pid" 2>/dev/null; then
+    echo "killing previous watch (pid $old_pid)"
+    kill "$old_pid" 2>/dev/null || true
+    for _ in {1..50}; do
+      kill -0 "$old_pid" 2>/dev/null || break
+      sleep 0.1
+    done
+  fi
+  rm -f "$PID_FILE"
+fi
+
+tsdown --watch &
+child=$!
+echo "$child" > "$PID_FILE"
+
+cleanup() {
+  kill "$child" 2>/dev/null || true
+  if [[ "$(cat "$PID_FILE" 2>/dev/null)" == "$child" ]]; then
+    rm -f "$PID_FILE"
+  fi
+}
+trap cleanup EXIT INT TERM
+
+wait "$child"