ExecutionContext

Installs only integration-owned ambient state. Each invocation returns its own restoration token. Close restores the preceding state on that installation's thread. Installations can nest and run concurrently, including overlapping coroutine update/restore calls; never share a latest-token slot. If install partially mutates state and throws, the provider must restore its partial mutation itself. Mosaic cannot reconstruct unknown state. Neither install nor close may block on Mosaic progress.