（gemini3pro）Specify 运行多次失败的案例：addBlock 的 new Mesh() → Chunk instanced Meshing 优化

模型：gemini3 pro

> 需求：为了提高运行时 fps，将用户新添加的方块从原来的简单new Mesh()的方式重构为纳入 InstancedMesh 管理，从而大幅减少新生成方块的 draw call次数，复杂度降为O(n)。

第一次优化，直接告诉claude我的需求：执行失败，虽然实现了，但是实现逻辑非常复杂，引入的bug太多，改不过来：https://github.com/jayli/mc-lite/tree/003-greedy-meshing

The issue names no files, tests, or entry points. Start by reviewing the referenced 003-greedy-meshing branch and locating the current new Mesh() path; define the affected chunk-meshing behavior and a way to verify that instanced meshing reduces draw calls without introducing regressions.

Written by the indexing model from the issue text.