feat: LOD 网格分级——meshopt 解析期简化 + 按距离/级联选级

vendored meshoptimizer 1.2（submodule 环境故障改直接 vendor）。Mesh
多 LOD：各级索引连续拼进同一 index buffer（共享顶点，LodRange 区间
选择），解析期 meshopt_simplify 生成链（LOD0=原始，每级目标减半，
相对误差 1e-2，小网格复用上级）；都在 worker 线程，加载零额外卡顿。

SceneRenderer 按 (mesh, LOD) 聚合 batch（lookup key = 指针<<8|lod）：
主视图按屏占比近似 radius/dist 选级（阈值 10/20 倍半径）；阴影级联
按级联序降级（越远越简）。Draw 按 batch.lod 选索引区间。

A/B 对比（100 背包 10×10，RTX 5060 Laptop，vsync off）：
- 纯背包：LOD 开 93-95 fps / 关 90-107 fps —— 无显著差异
- 加 SunTemple：LOD 开 52-56 / 关 51-55 —— 无显著差异
结论：当前场景瓶颈不在顶点数（5060 顶点吞吐富余），LOD 基础设施
保留（几何复杂资产/低端 GPU/更大场景生效），帧率问题转向 Tracy
GPU 时间线定位真实瓶颈。

坑：LodRange 记录"LOD 不足复用最后一级"时 first 不得继续累加
（buffer 无重复数据，累加必越界——VUID-08798 抓的）。

Lib/meshoptimizer/gltf/json.cpp

@@ -0,0 +1,92 @@
+// This file is part of gltfpack; see gltfpack.h for version/license details
+#include "gltfpack.h"
+
+#include <float.h>
+#include <math.h>
+#include <stdio.h>
+
+void comma(std::string& s)
+{
+	char ch = s.empty() ? 0 : s[s.size() - 1];
+
+	if (ch != 0 && ch != '[' && ch != '{')
+		s += ",";
+}
+
+void append(std::string& s, size_t v)
+{
+	char buf[32];
+	snprintf(buf, sizeof(buf), "%zu", v);
+	s += buf;
+}
+
+void append(std::string& s, float v)
+{
+	// sanitize +-inf to +-FLT_MAX and NaN to FLT_MAX
+	// it would be more consistent to use null for NaN but that makes JSON invalid, and 0 makes it hard to distinguish from valid values
+	float sv = fabsf(v) < FLT_MAX ? v : (v < 0 ? -FLT_MAX : FLT_MAX);
+
+	char buf[64];
+	snprintf(buf, sizeof(buf), "%.9g", sv);
+	s += buf;
+}
+
+void append(std::string& s, const char* v)
+{
+	s += v;
+}
+
+void append(std::string& s, const std::string& v)
+{
+	s += v;
+}
+
+void append(std::string& s, const float* data, size_t count)
+{
+	s += '[';
+	for (size_t i = 0; i < count; ++i)
+	{
+		if (i != 0)
+			s += ',';
+		append(s, data[i]);
+	}
+	s += ']';
+}
+
+void appendJson(std::string& s, const char* data)
+{
+	enum State
+	{
+		None,
+		Escape,
+		Quoted
+	} state = None;
+
+	for (const char* it = data; *it; ++it)
+	{
+		char ch = *it;
+
+		// whitespace outside of quoted strings can be ignored
+		if (state != None || !isspace(ch))
+			s += ch;
+
+		// the finite automata tracks whether we're inside a quoted string
+		switch (state)
+		{
+		case None:
+			state = (ch == '"') ? Quoted : None;
+			break;
+
+		case Quoted:
+			state = (ch == '"') ? None : (ch == '\\' ? Escape : Quoted);
+			break;
+
+		case Escape:
+			state = Quoted;
+			break;
+
+		default:
+			assert(!"Unexpected parsing state");
+		}
+	}
+}