Supercache dev extern mock clean

[yyyuanhao426-hash]

Description

将 UbDiag 从 Mooncake 编译期硬依赖（find_package(REQUIRED)）改为三层 CMake 自动回退机制，确保用户无论是否安装 UbDiag 均可编译 Mooncake。

新增 extern/ubdiag git submodule，指向 v0.5.1
新增 mooncake-common/FindUbDiag.cmake，实现 Layer 1 submodule → Layer 2 system → Layer 3 mock 三层分发
新增 mooncake-common/ubdiag-mock/ubdiag/auto_perf.h header-only mock，全签名对齐真实 UbDiag v0.5.1（含模板构造函数、Abandon、析构、PerfLevel:uint8_t）
mooncake-transfer-engine/src/CMakeLists.txt、mooncake-store/src/CMakeLists.txt、mooncake-integration/CMakeLists.txt 三处将 find_package(UbDiag REQUIRED) 替换为 include(FindUbDiag.cmake)
新增 mooncake-common/ubdiag-mock/README.md 集成说明文档

Module

• Transfer Engine (mooncake-transfer-engine)
• Mooncake Store (mooncake-store)
• Mooncake EP (mooncake-ep)
• Mooncake PG (mooncake-pg)
• Integration (mooncake-integration)
• P2P Store (mooncake-p2p-store)
• Python Wheel (mooncake-wheel)
• Common (mooncake-common)
• Mooncake RL (mooncake-rl)
• CI/CD
• Docs
• Other

Type of Change

• Bug fix
• New feature
• Refactor
• Breaking change
• Documentation update
• Performance improvement
• Other

How Has This Been Tested?

Test commands:

# Example: bash scripts/run_ci_test.sh

Test results:

• Unit tests pass
• Integration tests pass (if applicable)
• Manual testing done (describe below)

Checklist

• I have performed a self-review of my own code
• I have formatted my code using ./scripts/code_format.sh
• I have run pre-commit run --all-files and all hooks pass
• I have updated the documentation (if applicable)
• I have added tests to prove my changes are effective
• For changes >500 LOC: I have filed an RFC issue

AI Assistance Disclosure

• No AI tools were used
• AI tools were used (specify below)