feat: Replace Twig with native PHP templates

- Remove twig/twig dependency from composer.json
- Convert all 25 Twig templates to native PHP templates
- New render() method in Plugin.php using PHP include with output buffering
- New render_partial() helper method for including partials
- Templates support theme overrides via fedistream/ directory
- Reduced plugin size by eliminating Twig and its dependencies

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

templates/shortcodes/artists-grid.php

@@ -0,0 +1,36 @@
+<?php
+/**
+ * Artists grid shortcode template.
+ *
+ * @package WP_FediStream
+ *
+ * @var array  $posts   Array of artist data.
+ * @var int    $columns Number of columns.
+ * @var string $title   Section title.
+ */
+
+// Prevent direct file access.
+if ( ! defined( 'ABSPATH' ) ) {
+    exit;
+}
+
+$plugin  = \WP_FediStream\Plugin::get_instance();
+$columns = $columns ?? 4;
+?>
+<div class="fedistream-shortcode fedistream-shortcode--artists">
+    <?php if ( ! empty( $title ) ) : ?>
+        <h3 class="fedistream-shortcode__title"><?php echo esc_html( $title ); ?></h3>
+    <?php endif; ?>
+
+    <?php if ( ! empty( $posts ) && is_array( $posts ) ) : ?>
+        <div class="fedistream-grid fedistream-grid--artists fedistream-grid--cols-<?php echo esc_attr( $columns ); ?>">
+            <?php foreach ( $posts as $post ) : ?>
+                <?php echo $plugin->render_partial( 'partials/card-artist', array( 'post' => $post ) ); // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped ?>
+            <?php endforeach; ?>
+        </div>
+    <?php else : ?>
+        <div class="fedistream-empty">
+            <p><?php esc_html_e( 'No artists found.', 'wp-fedistream' ); ?></p>
+        </div>
+    <?php endif; ?>
+</div>