Move ai-stack and paintplus vendored source under vendor/

Matches the existing vendor/easy-asterisk convention (used by
services/asterisk.sh) instead of two one-off top-level directories that
cluttered the repo root and didn't look like anything else next to
setup.sh, lib/, services/, extras/. Only the two services' own SRC_DIR
path resolution and header comments needed updating — nothing else in
the repo referenced the old ./ai-stack / ./paintplus paths.

Also documents vendor/ in README.md's Layout section.

vendor/paintplus/frontend/src/js/actions/select-next-layer.js

@@ -0,0 +1,33 @@
+import app from './../app.js';
+import config from './../config.js';
+import { Base_action } from './base.js';
+
+export class Select_next_layer_action extends Base_action {
+	constructor(reference_layer_id) {
+		super('select_next_layer', 'Select Next Layer');
+		this.reference_layer_id = reference_layer_id;
+		this.old_config_layer = null;
+	}
+
+	async do() {
+		super.do();
+		const next_layer = app.Layers.find_next(this.reference_layer_id);
+		if (!next_layer) {
+			throw new Error('Aborted - Next layer to select not found');
+		}
+		this.old_config_layer = config.layer;
+		config.layer = next_layer;
+
+		app.Layers.render();
+		app.GUI.GUI_layers.render_layers();
+	}
+
+	async undo() {
+		super.undo();
+		config.layer = this.old_config_layer;
+		this.old_config_layer = null;
+
+		app.Layers.render();
+		app.GUI.GUI_layers.render_layers();
+	}
+}