Security audit fixes: fn() whitelist, escaping, and performance (v0.1.4)

- WooCommerceExtension: ALLOWED_FUNCTIONS whitelist for fn() Twig function
- Notice templates: data attributes use wp_kses_post instead of raw
- Search form: esc_attr on search query value attribute
- Per-request ContextBuilder caching via static variable
- Shared wc_bootstrap_render_in_page_shell() helper (DRY)
- Removed unused WC_BOOTSTRAP_VERSION and WC_BOOTSTRAP_URL constants

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/product-searchform.html.twig

@@ -21,7 +21,7 @@
                id="{{ field_id }}"
                class="form-control"
                placeholder="{{ __('Search products…') }}"
-               value="{{ get_search_query() }}"
+               value="{{ get_search_query()|esc_attr }}"
                name="s" />
         <button type="submit" class="btn btn-outline-primary" aria-label="{{ __('Search') }}">
             <i class="bi bi-search" aria-hidden="true"></i>