Add a build_time meson option (string, default '') sibling to the
existing commit_id option, threaded project-wide via the same
add_project_arguments() call that already defines VERSION_COMMIT_ID --
one call, one mechanism, no new plumbing. No consumer wired yet (later
tasks); a plain meson setup with neither option set still configures
cleanly with both defines resolving to empty string.
Task 1/13 of build-time-versioning (spec: .kiro/specs/build-time-versioning/).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>