build(studio): make studio skips the frontend and kernel steps when nothing they read is newer than what they made

STUDIO_REBUILD=1 forces both. The comparison is a small Node script because the launcher runs under a Git Bash that may have no GNU find.

desktop/electron/packaging/stale.js

@@ -0,0 +1,35 @@
+"use strict";
+const fs = require("node:fs");
+const path = require("node:path");
+
+// Exit 0 when the artifact is missing or any file under the given paths is
+// newer than it, 1 when it is up to date. run-studio.sh asks this before
+// rebuilding; the shell has no portable `find` on every platform it runs on.
+const [artifact, ...inputs] = process.argv.slice(2);
+if (!artifact) process.exit(2);
+
+let built;
+try {
+  built = fs.statSync(artifact).mtimeMs;
+} catch {
+  process.exit(0);
+}
+
+const SKIP = new Set(["node_modules", ".git", "dist"]);
+
+function newer(entry) {
+  let info;
+  try {
+    info = fs.statSync(entry);
+  } catch {
+    return false;
+  }
+  if (!info.isDirectory()) return info.mtimeMs > built;
+  for (const name of fs.readdirSync(entry)) {
+    if (SKIP.has(name)) continue;
+    if (newer(path.join(entry, name))) return true;
+  }
+  return false;
+}
+
+process.exit(inputs.some(newer) ? 0 : 1);

desktop/electron/run-studio.sh

@@ -12,11 +12,29 @@ if [ ! -d "$here/node_modules" ]; then
   (cd "$here" && pnpm install --frozen-lockfile)
 fi
 
-echo "==> frontend"
-(cd "$here/../frontend-next" && pnpm install --frozen-lockfile >/dev/null && pnpm build >/dev/null)
+# A step is skipped when nothing it reads is newer than what it produced;
+# STUDIO_REBUILD=1 forces every step.
+stale() {
+  [ -n "${STUDIO_REBUILD:-}" ] || node "$here/packaging/stale.js" "$@"
+}
 
-echo "==> kernel"
-(cd "$here" && pnpm build:host)
+root="$here/../.."
+web="$here/../frontend-next"
+host=$(ls "$here"/bin/reasonix-studio-host* 2>/dev/null | head -n 1 || true)
+
+if stale "$web/dist/index.html" "$web/src" "$web/public" "$web/index.html" "$web/package.json" "$web/pnpm-lock.yaml" "$web/vite.config.ts" "$web/tsconfig.json"; then
+  echo "==> frontend"
+  (cd "$web" && pnpm install --frozen-lockfile >/dev/null && pnpm build >/dev/null)
+else
+  echo "==> frontend (up to date)"
+fi
+
+if [ -z "$host" ] || stale "$host" "$root/internal" "$root/cmd" "$root/go.mod" "$root/go.sum" "$here/packaging"; then
+  echo "==> kernel"
+  (cd "$here" && pnpm build:host)
+else
+  echo "==> kernel (up to date)"
+fi
 
 echo "==> studio"
 cd "$here" && exec pnpm start