Phase 0.63 React island boundary

React remains an optional, Experimental interoperability path. Hedron core does not import or bundle React. react_island_recipe() exposes the pinned adapter metadata, and react_island_host() renders a server-owned fallback inside a CSP-auditable host with cleanup and SSR markers.

An application may attach the pinned adapter at the host boundary, but the fallback must remain complete and usable when JavaScript is absent or the adapter is blocked by policy. The recipe is intentionally isolated: it does not change the component registry, execute callbacks, or make React a runtime dependency of the core package.