[M3] add codegen::cmake_lists

CHANGELOG.md

@@ -70,6 +70,14 @@ All notable changes to cargoxx will be documented in this file.
   not show `buildInputs`; we add one between `nativeBuildInputs` and the
   `env.NIX_CFLAGS_COMPILE` block as the natural slot for the deps that
   TECH_SPEC §10 says we splice in.
+- `codegen::cmake_lists(in) -> std::string` per `SPEC.md` §8 and
+  `TECH_SPEC.md` §9: `find_package` per dep, optional library target
+  with module units + private impl sources, primary binary
+  `<pkg>_bin` linked against the library, additional binaries from
+  `src/bin/`, tests with `enable_testing()` + `add_test`, examples,
+  and `[build]` honoring `warnings_as_errors` and `sanitizers`. Source
+  paths emitted relative to `build/` (i.e. prefixed with `../`).
+  Output is deterministic. `tests/codegen_cmake.cpp` covers 11 cases.
 - SQLite overlay: `Database::open(overlay_path)` now opens (and creates,
   if missing) a per-user `linkdb.sqlite` cache, applying the schema from
   `SPEC.md` §9 idempotently. Default path is