fix: terrain descriptor pool leak, minimap quest markers, and item comparison UI

- terrain_renderer: add FREE_DESCRIPTOR_SET_BIT flag and vkFreeDescriptorSets
  in destroyChunkGPU so material descriptor sets are returned to the pool;
  prevents GPU device lost from pool exhaustion near populated areas
- game_screen: fix projectToMinimap to use the exact inverse of the minimap
  shader transform so quest objective markers appear at the correct position
  and orientation regardless of camera bearing
- inventory_screen: fix item comparison tooltip to not compare equipped items
  against themselves (character screen); add item level diff line; show (=)
  indicator when stats are equal rather than bare value which looked identical
  to the item's own tooltip

src/rendering/terrain_renderer.cpp

@@ -89,6 +89,7 @@ bool TerrainRenderer::initialize(VkContext* ctx, VkDescriptorSetLayout perFrameL
 
     VkDescriptorPoolCreateInfo poolInfo{};
     poolInfo.sType = VK_STRUCTURE_TYPE_DESCRIPTOR_POOL_CREATE_INFO;
+    poolInfo.flags = VK_DESCRIPTOR_POOL_CREATE_FREE_DESCRIPTOR_SET_BIT;
     poolInfo.maxSets = MAX_MATERIAL_SETS;
     poolInfo.poolSizeCount = 2;
     poolInfo.pPoolSizes = poolSizes;
@@ -1034,6 +1035,10 @@ void TerrainRenderer::destroyChunkGPU(TerrainChunkGPU& chunk) {
         destroyBuffer(allocator, ab);
         chunk.paramsUBO = VK_NULL_HANDLE;
     }
+    // Return material descriptor set to the pool so it can be reused by new chunks
+    if (chunk.materialSet && materialDescPool) {
+        vkFreeDescriptorSets(vkCtx->getDevice(), materialDescPool, 1, &chunk.materialSet);
+    }
     chunk.materialSet = VK_NULL_HANDLE;
 
     // Destroy owned alpha textures (VkTexture::~VkTexture is a no-op, must call destroy() explicitly)