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/chat-apply.spec.tsx

@@ -32,12 +32,13 @@ async function bench() {
     current: undefined,
     phase: 'ready',
   })
+  const absentInfo = { sessionId: undefined, hooks: {}, props: {} }
   const sessionsFake = {
     list: listStore,
     binding: vi.fn(),
     scope: () => undefined,
     provideInfo: () => undefined,
-    maybeProvideInfo: () => ({ hooks: {}, props: {} }),
+    currentProvide: { getSnapshot: () => absentInfo, subscribe: () => () => {} },
     provide: vi.fn(() => () => {}),
     create: vi.fn(),
     open: vi.fn(),