feat(phase2): shared common-form layer + CommonWidgetFields trait for all widgets

- Extract Zibll CSF_Widget::params() common-field parity into
  inc/modules/widget-common.php (required by widgets-register.php to keep
  both files <1000 lines): subtitle, title_style(simple/big), title_highlight
  +color, title_link x3, title_link_style colors, sidebar_affix, show_type
  (all/only_pc/only_sm), show_id_type(show/hide), show_ids, animation_in,
  animation_repeat, layout_pin_pc/m, layout_bg(white/dark). Adds
  normalize_link/normalize_bg helpers, affix_attrs, common_wrapper_attr,
  render_title, animation_class. Back-compat normalizes legacy tokens
  (in_affix->sidebar_affix, pc/only_pc, include/exclude->show/hide).
- Add src/AKSRT/Widgets/Traits/CommonWidgetFields.php trait
  (render_common_form_section + update_common_fields).
- Wire all 57 widgets to use the trait; migrate the 16 hand-rolled in_affix
  implementations to shared sidebar_affix (fixes 6 latent bugs where
  in_affix was never persisted). SearchWidget intentionally skips the shared
  form section (its business show_type collides) but adopts the affix helper.
- widget_display_callback parity verified via tmp assertion script.

Refs .specs/plans/zibll-widgets-1to1.md Phase 2 (US1/US5)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AKSRT/Widgets/Traits/CommonWidgetFields.php

@@ -0,0 +1,73 @@
+<?php
+/**
+ * CommonWidgetFields trait — single seat-belt for shared widget form/update.
+ *
+ * Every AKSRT widget `use`s this trait so the Zibll common-field family
+ * (title block, display rules, affix, animation, layout bg) is declared and
+ * persisted in exactly one place: inc/modules/widget-common.php.
+ *
+ * Widgets keep only their business fields; the trait wires the shared section
+ * into form() and folds the shared keys into update().
+ *
+ * @package AKSRT\Widgets\Traits
+ */
+
+namespace AKSRT\Widgets\Traits;
+
+defined('ABSPATH') || exit;
+
+trait CommonWidgetFields {
+
+	/**
+	 * Whether this widget should render the shared common-form section.
+	 *
+	 * Override and return false for pure-display widgets with no admin form.
+	 * Default: true (every widget gets the shared block).
+	 *
+	 * @return bool
+	 */
+	protected function use_common_form(): bool {
+		return true;
+	}
+
+	/**
+	 * Render the shared common-form section at the end of a widget's form().
+	 *
+	 * Call from form(): render business fields, then `$this->render_common_form_section($instance);`.
+	 *
+	 * @param array<string,mixed> $instance Widget instance.
+	 */
+	protected function render_common_form_section(array $instance): void {
+		if (!$this->use_common_form()) {
+			return;
+		}
+		if (function_exists('aksrt_widget_render_common_form')) {
+			aksrt_widget_render_common_form($instance);
+		}
+	}
+
+	/**
+	 * Merge shared common fields into a widget's update() result.
+	 *
+	 * Usage in update():
+	 *   return $this->update_common_fields($new_instance, $old_instance, [
+	 *       'title' => sanitize_text_field(...),
+	 *       ...business fields...
+	 *   ]);
+	 *
+	 * @param array<string,mixed> $new_instance New submitted instance.
+	 * @param array<string,mixed> $old_instance Previous instance.
+	 * @param array<string,mixed> $business     Sanitized business fields to keep.
+	 * @return array<string,mixed>
+	 */
+	protected function update_common_fields(array $new_instance, array $old_instance, array $business = []): array {
+		if (!$this->use_common_form()) {
+			return $business;
+		}
+		if (function_exists('aksrt_widget_update_common')) {
+			$common = aksrt_widget_update_common($new_instance, $old_instance);
+			return array_merge($business, $common);
+		}
+		return $business;
+	}
+}