feat(json): add high-performance C++20 DOM parser

[qicosmos]

Summary

• add a C++20 read-only JSON DOM with owned, borrowed, reusable, lazy-number, and trusted-input parsing modes
• optimize the indexed stage-2 array/object continuation paths and optional AVX2 scanning
• add correctness, malformed-input, UTF-8, deep-nesting, duplicate-key, and corpus coverage
• extend the JSON benchmark with filtered Iguana, simdjson, yyjson, and nlohmann runs
• document the public API and add safe CMake defaults for optional benchmark dependencies

Why

Iguana needed a reusable DOM parsing path for workloads that require dynamic JSON access while retaining competitive throughput and avoiding per-parse allocations where possible.

Performance

On the controlled fixed-core corpus run, Iguana reached 2.271 GiB/s versus simdjson at 2.466 GiB/s, a 7.81% gap. The retained continuation-path optimization improved array strings by 15.54%, object strings by 8.39%, and array numbers by 20.77%.

Validation

• MSVC default instruction set: 4267/4267 assertions
• MSVC AVX2: 4272/4272 assertions
• AddressSanitizer: 4267/4267 assertions
• full CTest suite: 12/12 tests
• Clang 18 direct build: 4267/4267 assertions
• JSON benchmark CLI smoke test
• clang-format and git diff --check

[qicosmos]

主要问题：

1. 编译错误风险：benchmark/json_benchmark.cpp 约 275 行起多处使用 ScopedTimer timer(name.data(), ns);，但 diff 中 ScopedTimer 构造函数仍显示只接受 const char*。若未新增重载会直接编译失败。

2. CMake 顺序/目标风险：CMakeLists.txt 约 136 行对 json_benchmark 调用 set_target_properties/target_compile_options，需确认该 target 一定已创建，否则配置失败。

3. AVX2 选项安全性：约 145 行仅按 CMAKE_SYSTEM_PROCESSOR 判断，不代表运行机器支持 AVX2。开启后生成二进制在非 AVX2 CPU 上会非法指令，建议文档明确或做运行时分发。

4. benchmark 公平性：DOM parse 每轮构造/销毁 document，而 reuse 版本复用容量；结果需明确区分，否则容易误导。

5. 代码风格：volatile size_t json_parse_sink 用于防优化可以接受，但 json_parse_sink = json_parse_sink + value 不必要，建议用更标准的 benchmark 防优化方式。