[M4] add exec::run subprocess wrapper

CHANGELOG.md

@@ -78,6 +78,15 @@ 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.exec`: `ExecResult`, `ExecOptions`, and
+  `run(program, args, opts)` — argv-only subprocess wrapper around
+  reproc 14.2.4. Captures stdout/stderr (or inherits stdio when
+  `opts.inherit_stdio` is set), supports `cwd`, `env_overrides`, and a
+  `timeout` (enforced via `reproc_options.deadline` so drains return
+  `REPROC_ETIMEDOUT` instead of blocking on long-lived streams). On
+  destruction reproc terminates and then kills any still-running child.
+  Returns `ExecToolNotFound` for `ENOENT` and `ExecCommandFailed` for
+  other failures including timeouts. `tests/exec_run.cpp` covers 9 cases.
 - `cargoxx build --no-build` end-to-end. Reads `Cargoxx.toml`,
   discovers the layout, opens the curated linkdb, resolves a `Recipe`
   per manifest dep, synthesizes a fresh `Cargoxx.lock`, and writes