[M1] add util::Error, Result<T>, format()

CHANGELOG.md

@@ -12,3 +12,10 @@ All notable changes to cargoxx will be documented in this file.
   builds an empty `cargoxx` binary.
 - `.clang-format` (LLVM, 100-column) and `.gitignore`.
 - `SPEC.md`, `TECH_SPEC.md`.
+- M1 foundation in `cargoxx.util`: `ErrorCode` (numbers per `TECH_SPEC.md` §4),
+  `Error`, `Result<T> = std::expected<T, Error>`, and `format(Error)` rendering
+  `error[Ennnn]: ...` with optional `--> path:line:col` and `hint:` lines.
+  `format` lives in `src/util/error.cpp` as a module implementation unit.
+- `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`.