Cover context lookup across TU keys

Co-authored-by: skypjack <1812216+skypjack@users.noreply.github.com>

Author: Copilot

test/entt/entity/registry.cpp

@@ -2372,9 +2372,12 @@ TEST_F(Registry, ContextTypeMatchingAcrossTranslationUnits) {
     entt::registry registry{};
     auto *const expected = test::registry_context_insert(registry);
     const auto *const value = registry.ctx().find<test::registry_context_type *>();
+    const auto *const hinted = registry.ctx().find<test::registry_context_type *>(0u);
 
     ASSERT_NE(value, nullptr);
     ASSERT_EQ(*value, expected);
+    ASSERT_NE(hinted, nullptr);
+    ASSERT_EQ(*hinted, expected);
 }
 
 TEST_F(Registry, ContextHint) {

test/entt/entity/registry_context.cpp

@@ -7,7 +7,9 @@ registry_context_value<Type>::registry_context_value() = default;
 
 template<typename Type>
 registry_context_value<Type> *registry_context_value<Type>::insert(entt::registry &registry) {
-    registry.ctx().emplace<registry_context_value<Type> *>(this);
+    auto &ctx = registry.ctx();
+    ctx.emplace<registry_context_value<Type> *>(this);
+    ctx.emplace_as<registry_context_value<Type> *>(0u, this);
     return this;
 }