refactor(build): move vendored deps to third_party

third_party/README.md

@@ -0,0 +1,16 @@
+# Third-Party Dependencies
+
+All vendored dependencies in this repo live under `third_party/`.
+
+Dependencies:
+- `CLI11` (git submodule): command-line parsing, target `CLI11::CLI11`
+- `proxy` (git submodule): proxy/rtti headers, target `msft_proxy4::proxy`
+- `tomlplusplus` (vendored source drop): TOML parsing, target `tomlplusplus::tomlplusplus`
+- `mcap` (vendored source drop): MCAP headers, target `mcap::mcap`
+
+Bootstrap:
+- `git submodule sync --recursive`
+- `git submodule update --init --recursive`
+
+Rule:
+- New vendored dependencies must live under `third_party/` and be exposed through `third_party/CMakeLists.txt`.