fix: make SM Lab cascade inspector keyboard focusable

Fixes #102

Author: georgeolaru

src/Lab/ShowcaseRenderer.php

@@ -403,6 +403,7 @@ class="sm-lab-signal-cascade__node sm-lab-signal-cascade__node--<?php echo esc_a
 			data-sm-lab-cascade-resolved-grade="<?php echo esc_attr( (string) $node['resolved_grade'] ); ?>"
 			data-sm-lab-cascade-text-grade="<?php echo esc_attr( (string) $node['text_grade'] ); ?>"
 			data-sm-lab-cascade-active="<?php echo esc_attr( $node['active'] ? 'true' : 'false' ); ?>"
+			tabindex="0"
 			style="<?php echo esc_attr( $node['style'] ); ?>"
 		>
 			<span class="sm-lab-signal-cascade__signal">

tests/phpunit/Unit/Lab/ShowcaseRendererTest.php

@@ -61,6 +61,7 @@ public function test_render_outputs_unique_value_showcase_sections(): void {
 		$this->assertStringContainsString( 'data-sm-lab-cascade-node="second-inner"', $html );
 		$this->assertStringContainsString( 'data-sm-lab-cascade-parent="inner"', $html );
 		$this->assertStringContainsString( 'data-sm-lab-cascade-signal="3"', $html );
+		$this->assertStringContainsString( 'tabindex="0"', $html );
 		$this->assertStringContainsString( 'data-sm-lab-cascade-active="true"', $html );
 		$this->assertStringContainsString( 'data-sm-lab-cascade-value="parent"', $html );
 		$this->assertStringContainsString( '--sm-lab-cascade-surface-color', $html );