[sanchuanhehe]

A polling endpoint or session retained its shared-mailbox backends without a way to reclaim them, even after the platform had stopped the peer. Add explicit unsafe reclaim methods that consume the endpoint or session and return both backends and the authority policy.

Reclaim requires quiescence, no outstanding references or accesses, cache reconciliation, and explicit acceptance of losing any staged receive. Session invalidation alone does not satisfy these requirements. Add a host test covering an invalidated session with a staged message and both backend destructors.

Validation: 29 all-feature host tests and host Clippy passed from an independent remote checkout; RISC-V Clippy passed locally with build-std. No physical peer or cache behavior was exercised.