refactor: eliminate global static state in Physics/Spine WASM modules

[esengine]

Problem

Physics 和 Spine 的 C++ binding 层大量使用全局静态变量：

• PhysicsModuleEntry.cpp:25-34 — g_worldId, g_entityToBody, g_entityToShape, g_fixedTimestep, g_accumulator
• SpineModuleEntry.cpp:106+ — g_nextSkeletonId, g_nextInstanceId
• Renderer.cpp — s_current_blend_

Impact

• 场景切换时状态残留，导致 Play Mode 退出后物理/Spine 行为异常
• 无法支持多世界物理模拟
• WASM 模块无法安全重载

Solution

将全局状态迁移到 EngineContext 或独立的 per-world context 结构体，通过指针传入 binding 函数。

[esengine]

All module-level global state has been encapsulated:

• Physics: 14 globals → PhysicsContext struct with reset()
• Spine: 8 globals → SpineContext struct with reset()
• Renderer: s_current_blend_ → RenderCommand::currentBlend_ class member
• s_materialCache: already function-encapsulated, no change needed

Scene switching now cleanly resets via context reset methods.