feat(gameplay): spawn creatures during chunk generation

Add creature spawning to chunk generation with a configurable
SpawnConfig, including a default pig spawn. Spawning occurs in
the final generation phase and registers entities through the
server world's entity manager.

Also destroy entities when their chunk is unloaded to prevent
orphaned entities and expose the entity manager from ServerWorld.

include/Cubed/gameplay/chunk_generator.hpp

@@ -50,6 +50,8 @@ public:
     void generate_cave();
     void generate_river();
 
+    void spawn_creature();
+
 private:
     static inline std::atomic<bool> is_init{false};
     static inline unsigned m_generator_seed{0};

include/Cubed/gameplay/creatures/spawn.hpp

@@ -0,0 +1,20 @@
+#pragma once
+#include "Cubed/gameplay/biome.hpp"
+
+#include <array>
+#include <span>
+#include <string_view>
+namespace Cubed {
+struct SpawnConfig {
+    std::string_view name;             // factory key, e.g. "cubed:pig"
+    std::span<const BiomeType> biomes; // allowed biomes
+    float probability = 0.0f;          // per chunk spawn probability
+    unsigned max_spawn_count = 0;      // per chunk_max_spawn_sum
+};
+
+namespace SpawnDefaults {
+constexpr std::array<BiomeType, 2> PIG_BIOMES{BiomeType::PLAIN,
+                                              BiomeType::FOREST};
+constexpr SpawnConfig PIG{"cubed:pig", PIG_BIOMES, 0.02f, 3};
+} // namespace SpawnDefaults
+} // namespace Cubed

include/Cubed/gameplay/server_entity_manager.hpp

@@ -15,7 +15,7 @@ public:
     void init();
     void update();
     // not thread safe
-    void add_entity(std::string_view name, const glm::vec3& pos);
+    void add_entity(std::string_view name, const glm::vec3& world_pos);
     void destory(EntityID id);
     void handle_player_login(std::shared_ptr<Session> session);
 

include/Cubed/gameplay/server_world.hpp

@@ -95,6 +95,7 @@ public:
     std::vector<std::shared_ptr<Session>> get_all_session() const;
 
     uint32_t get_chunk_ref_count(const glm::vec3& pos) const;
+    ServerEntityManager& entity_manager();
 
     int get_block(const glm::ivec3& block_pos) const override;
     bool is_solid(const glm::ivec3& block_pos) const override;