Build configuration — reference-implementation design notes

Scope: the build configuration of the C++23 reference implementation. Not normative. Target: the sources under core/. Companions: ../../reference/10-module-catalog.md says what each module is responsible for, at standard level; ../../reference/12-deployment-profiles.md says which combinations serve which deployments. Neither is allowed to name a CMake option, and that is the gap these notes fill. Nothing here is normative.

The reference suite is deliberately not a build guide, so a reader who wants to know which knobs exist, what each costs on their target, and which constants are off-limits has nowhere in it to look. That question is entirely implementation-specific — a second implementer needs none of it — but an integrator needs all of it.

File	Topic
00-configuration-space.md	Every configuration axis the build exposes, its per-target cost, and the constants that are not axes.