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/partials/card-artist.php

@@ -0,0 +1,47 @@
+<?php
+/**
+ * Artist card partial template.
+ *
+ * @package WP_FediStream
+ *
+ * @var array $post Artist data array.
+ */
+
+// Prevent direct file access.
+if ( ! defined( 'ABSPATH' ) ) {
+    exit;
+}
+?>
+<article class="fedistream-card fedistream-card--artist">
+    <a href="<?php echo esc_url( $post['permalink'] ?? '' ); ?>" class="fedistream-card__link">
+        <div class="fedistream-card__image">
+            <?php if ( ! empty( $post['thumbnail'] ) ) : ?>
+                <img src="<?php echo esc_url( $post['thumbnail'] ); ?>" alt="<?php echo esc_attr( $post['title'] ?? '' ); ?>" loading="lazy">
+            <?php else : ?>
+                <div class="fedistream-card__placeholder fedistream-card__placeholder--artist">
+                    <svg viewBox="0 0 24 24" fill="currentColor"><path d="M12 12c2.21 0 4-1.79 4-4s-1.79-4-4-4-4 1.79-4 4 1.79 4 4 4zm0 2c-2.67 0-8 1.34-8 4v2h16v-2c0-2.66-5.33-4-8-4z"/></svg>
+                </div>
+            <?php endif; ?>
+        </div>
+        <div class="fedistream-card__content">
+            <h3 class="fedistream-card__title"><?php echo esc_html( $post['title'] ?? '' ); ?></h3>
+            <p class="fedistream-card__meta">
+                <span class="fedistream-card__type"><?php echo esc_html( $post['artist_type_label'] ?? '' ); ?></span>
+                <?php if ( ! empty( $post['location'] ) ) : ?>
+                    <span class="fedistream-card__location"><?php echo esc_html( $post['location'] ); ?></span>
+                <?php endif; ?>
+            </p>
+            <?php if ( isset( $post['album_count'] ) && $post['album_count'] > 0 ) : ?>
+                <p class="fedistream-card__stats">
+                    <?php
+                    printf(
+                        /* translators: %d: number of albums */
+                        esc_html( _n( '%d album', '%d albums', $post['album_count'], 'wp-fedistream' ) ),
+                        (int) $post['album_count']
+                    );
+                    ?>
+                </p>
+            <?php endif; ?>
+        </div>
+    </a>
+</article>