fix(client): publish current session provide bundle as one reactive projection

A provider roster change under a stable current id rematerialized every
scope's bundle but nothing notified React: SessionProvider resolved the
bundle from a current-id subscription only, so mounted entries kept the
obsolete hook/prop schema until an unrelated re-render. The sessions
service now owns an atomic currentProvide observable fed by both current
writes and roster changes; the renderer host exposes it as
sessions.provide, replacing the current/provideInfo/maybeProvideInfo
trio, and both providers subscribe to it.

packages/client/ui-conversation/tests/apply-inject.spec.tsx

@@ -87,12 +87,13 @@ async function bench() {
     }
   }
   const providers: TestProvider[] = []
+  const absentInfo = { sessionId: undefined, hooks: {}, props: {} }
   const sessionsFake = {
     list: listStore,
     binding: (id: SessionId) => ({ sessionId: id, session: sessionFake, ctx: mint(id) }),
     scope: (id: SessionId) => mint(id),
     provideInfo: () => undefined,
-    maybeProvideInfo: () => ({ hooks: {}, props: {} }),
+    currentProvide: { getSnapshot: () => absentInfo, subscribe: () => () => {} },
     provide: (descriptor: TestProvider) => { providers.push(descriptor); return () => {} },
     scopeOf,
     sessionOf: (actx: Context) => (scopeOf(actx) === undefined ? undefined : sessionFake),