[M4] cargoxx build invokes nix+cmake

CHANGELOG.md

@@ -78,6 +78,20 @@ All notable changes to cargoxx will be documented in this file.
   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.
+- `cargoxx build` (without `--no-build`) now drives the full build:
+  `nix develop -c cmake -B build/<profile> -S build -G Ninja
+  -DCMAKE_BUILD_TYPE=<Profile>` then `nix develop -c cmake --build`
+  with optional `--target`, both with `inherit_stdio = true` so the
+  user sees compilation output live. Non-zero cmake exit returns
+  `BuildCmakeFailed`. End-to-end verified: `cargoxx new myapp &&
+  cargoxx build` produces a working `build/debug/myapp` that prints
+  `Hello from myapp!`.
+- Generated `build/CMakeLists.txt` now opts into the experimental
+  `import std;` UUID + `CMAKE_CXX_MODULE_STD ON`, and sets
+  `CMAKE_CXX_EXTENSIONS ON` (deviation from SPEC §8 — required for
+  libc++'s std module to load without `module-file-config-mismatch`
+  on clang 21). Without these the templates from `cargoxx new` fail
+  to compile.
 - `cargoxx.exec`: `ExecResult`, `ExecOptions`, and
   `run(program, args, opts)` — argv-only subprocess wrapper around
   reproc 14.2.4. Captures stdout/stderr (or inherits stdio when