[M1] add manifest::parse

CHANGELOG.md

@@ -19,3 +19,9 @@ All notable changes to cargoxx will be documented in this file.
 - `Catch2 v3` wired through `flake.nix` (libc++-built override) and registered
   in CMake; `tests/util_error.cpp` covers six format cases via
   `catch_discover_tests`.
+- `cargoxx.manifest` public types (`Package`, `Dependency`, `BuildSettings`,
+  `Edition`, `Manifest`) and `parse(path)` returning `Result<Manifest>`,
+  backed by toml++ vendored as `third_party/toml.hpp`. Unknown keys in
+  `[package]` / `[build]` and unknown top-level keys are rejected; reserved
+  fields (`description`, `repository`, `[dev-dependencies]`, `[features]`,
+  `[workspace]`) are accepted. `tests/manifest_parse.cpp` covers 17 cases.