ADR 0019: Bounded renderer-independent runtime UI core

Status: Accepted for U0.

Ludus needs runtime HUD/menu layout and input ownership without a dependency on its desktop Qt editor or a renderer feature set that does not yet exist.

Introduce Ludus::Ui: value document descriptions, persistent stable-ID focus and pointer capture, transactional bounded layout resolution, clipped ordered solid-rectangle commands, and callback-free action results. Allocate explicitly at initialization, never while resolving or routing events. Keep platform, text/IME, accessibility, authoring and GPU adapters separate. Browser controls continue to use native DOM semantics; desktop authoring continues to use Qt.

The core is available in native and browser SDKs with a Base-only public header. General text/image rendering, controller navigation, modal scopes and authoring remain staged work. Sandbox's bounded fullscreen rectangle adapter proves the current core and will be replaced by an indexed-quad renderer when RHI resources permit it. It is not the general rendering strategy.

See the architecture and Gems review for alternatives, contracts, delivery gates and reasons not to adopt an XML condition hierarchy, global constraint solver or unmeasured cache graph in the first slice.