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-code-subcalls.spec.tsx

@@ -87,6 +87,9 @@ async function bench(snapshot: ConversationSnapshot) {
   // Provide-channel contributions land in this bundle the way the runtime
   // materializes them; the renderer host serves it through provideInfo.
   const provided: { hooks: Record<string, unknown>; props: Record<string, unknown> } = { hooks: {}, props: {} }
+  // Identity-stable currentProvide snapshot (uSES getSnapshot contract),
+  // materialized on first render after the provide contributions landed.
+  let infoCell: { sessionId: SessionId; hooks: Record<string, unknown>; props: Record<string, unknown> } | undefined
   const sessionsFake = {
     list,
     binding: (id: SessionId) => (id === SID
@@ -103,9 +106,10 @@ async function bench(snapshot: ConversationSnapshot) {
     provideInfo: (id: string) => (id === SID
       ? { sessionId: SID, hooks: { session, ...provided.hooks }, props: provided.props }
       : undefined),
-    maybeProvideInfo: (id: string | undefined) => (id === SID
-      ? { sessionId: SID, hooks: { session, ...provided.hooks }, props: provided.props }
-      : { hooks: provided.hooks, props: provided.props }),
+    currentProvide: {
+      getSnapshot: () => infoCell ??= { sessionId: SID, hooks: { session, ...provided.hooks }, props: provided.props },
+      subscribe: () => () => {},
+    },
     create: vi.fn(),
     open: vi.fn(),
   }