switchBackend consumes the backend; no way to re-install a caller-owned instance

[Yaraslaut]

Summary

Bridge::switchBackend takes a std::unique_ptr, i.e. it consumes the backend:

void switchBackend(std::unique_ptr<::morph::backend::detail::IBackend> newBackend);

That cannot express re-installing a backend instance the caller still owns.

Use case

An application that toggles between a remote and a local backend as connectivity comes
and goes typically holds one long-lived remote backend for the lifetime of the
process — it owns the socket, the reconnect timer and the pending-request state — and
swaps the Bridge between that instance and a local one:

auto remote = std::make_shared<SomeRemoteBackend>(url);
auto local  = std::make_shared<LocalBackend>(pool);

// Connection lost — fall back.
bridge.switchBackend(local);

// Reconnected — put the *same* remote instance back.
bridge.switchBackend(remote);   // cannot be expressed with unique_ptr

With the current signature the caller has to either give up ownership on the first
switch (and then has nothing to switch back to), or construct a fresh remote backend on
every transition — which throws away the connection state that made it worth keeping.

Suggested fix

Add a shared_ptr overload and let the unique_ptr one delegate to it:

void switchBackend(std::shared_ptr<::morph::backend::detail::IBackend> newBackend);

void switchBackend(std::unique_ptr<::morph::backend::detail::IBackend> newBackend) {
    switchBackend(std::shared_ptr<...>{std::move(newBackend)});
}

This is additive only — existing call sites are unaffected. The implementation already
converts to shared_ptr internally on its first line, so the overload mostly removes a
conversion the caller currently cannot perform.

Related: the rollback-on-partial-failure path inside switchBackend behaves the same
either way, since it operates on the converted pointer.

Happy to open a PR.