feat(search): add pagefind provider support

Add Pagefind indexing and browser search adapters behind a provider switch.

This lets prebuild generate either Stork or Pagefind search artifacts and lets the existing search UI run against Pagefind while preserving scoped filters, excerpts, and result metadata.

helpers/search/config.js

@@ -0,0 +1,18 @@
+export const defaultSearchProvider = 'pagefind'
+
+export const supportedSearchProviders = new Set([
+    'pagefind',
+    'stork'
+])
+
+export function getSearchProvider ( rawProvider = defaultSearchProvider ) {
+    const provider = ( rawProvider || defaultSearchProvider ).toLowerCase()
+
+    if ( supportedSearchProviders.has( provider ) ) {
+        return provider
+    }
+
+    console.warn(`Unknown search provider "${ provider }", falling back to "${ defaultSearchProvider }"`)
+
+    return defaultSearchProvider
+}