[MAINTENANCE] Replace vendored yaml-cpp with CMake FetchContent for cross-platform builds

[yunlishao-vibe]

Summary

yaml-cpp is currently fully vendored in `CommonLib/yaml-cpp/` (v0.7.0, ~500 files). This bloats the repo, makes version tracking manual, and ties the build system to Windows `.bat`/`.ps1` scripts — blocking progress on issue #65 (Linux compile support).

Proposed Changes

• Replace `CommonLib/yaml-cpp/` source tree with a `CMakeLists.txt`-based `FetchContent` declaration pinned to a specific tag
• Update `dependencies.yaml` to record the pinned yaml-cpp version (currently `0.7.0`, upstream is `0.8.0`)
• Update `scripts/dispatch/1_external_libraries.bat` (or replace with a CMake configure step) to fetch and build yaml-cpp automatically
• Remove the ~500 vendored files from git tracking

Motivation

• Prerequisite for Linux/cross-platform builds (issue [FEATURE] Enable both windows and linux compile #65) — the current `.bat` build system is Windows-only
• Reduces repo size significantly
• Version bumps become a one-line change in `dependencies.yaml`
• Follows modern CMake best practices

Acceptance Criteria

• `CommonLib/yaml-cpp/` source tree removed from git
• `FetchContent` (or equivalent) fetches yaml-cpp at the pinned version during CMake configure
• Windows build (`dispatch.bat`) still works after change
• `dependencies.yaml` updated with explicit yaml-cpp version
• Build documentation updated in `doc/BUILD.md`

Notes

yaml-cpp v0.7.0 → v0.8.0 has no breaking API changes for FIXS usage. The migration can bump to v0.8.0 at the same time.

Related: #65 (Linux compile), #131 (build system cleanup)