Feature: Recommendation Service

This commit introduces a RecommendationService that is used when indexing products.

The service is utilizing a recommendation rules interface, so many rules can be created and applied during indexing the products

src/Catalog/Contracts/RecommendationRule.php

@@ -0,0 +1,39 @@
+<?php
+
+namespace Modules\Core\Catalog\Contracts;
+
+use Illuminate\Support\Collection;
+use Lunar\Models\Product;
+
+/**
+ * One strategy for producing recommended products for a given product —
+ * e.g. same category, same tag, best sellers, random. Modules\Core\Catalog\
+ * Services\RecommendationService runs rules registered in
+ * config('catalog.recommendation_rules') in order, topping up from each
+ * successive rule until $limit distinct products are collected or every
+ * rule is exhausted (e.g. 3 from SameCategoryRule + 1 from RandomRule) —
+ * nothing here decides that accumulation itself; a store composes its own
+ * chain by ordering rules in config (e.g. [SameCategoryRule::class,
+ * RandomRule::class]).
+ *
+ * Returns raw Product models, not ProductService::list()'s locale-resolved
+ * array output — this runs at index time (ProductIndexer::toSearchableArray()),
+ * where "the current locale" isn't a meaningful concept the way it is for a
+ * storefront request. ProductIndexer resolves translated fields itself via
+ * translateAttribute(), same as it already does for the embedded `collections`
+ * field — same known index-time-locale tradeoff, not a new one.
+ */
+interface RecommendationRule
+{
+    /**
+     * $exclude carries $product's own id plus every id already picked by an
+     * earlier rule this call — RecommendationService never shows the same
+     * product twice even when two rules would both suggest it, and a rule
+     * shouldn't spend its $limit budget re-returning something already
+     * collected.
+     *
+     * @param  array<int>  $exclude
+     * @return Collection<int, Product> at most $limit products
+     */
+    public function recommend(Product $product, int $limit, array $exclude): Collection;
+}