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/selection-survival.spec.ts

@@ -11,6 +11,9 @@ import { createChatStore } from '../src/client/stores.ts'
 
 const sid = (s: string): SessionId => s as SessionId
 
+/** Identity-stable no-session bundle (uSES getSnapshot contract). */
+const ABSENT_INFO = { sessionId: undefined, hooks: {}, props: {} }
+
 interface Bench {
   slots: SlotsService
   chat: ReturnType<typeof createChatStore>
@@ -23,7 +26,10 @@ function bench(): Bench {
       ids: [], byId: {}, current: undefined, phase: 'ready',
     }),
     provideInfo: () => undefined,
-    maybeProvideInfo: () => ({ hooks: {}, props: {} }),
+    currentProvide: {
+      getSnapshot: () => ABSENT_INFO,
+      subscribe: () => () => {},
+    },
     provide: () => () => {},
   })
   ctx.provide('workspaces', {