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/web-react/tests/scoped-slots.spec.tsx

@@ -12,6 +12,7 @@ import { describe, expect, it, vi } from 'vitest'
 import { act, fireEvent, render } from '@testing-library/react'
 import { useEffect, type ReactNode } from 'react'
 import type { ActionsDecl, SlotEntryDef, SlotSpec, StoreHandle, StoredEntry } from '@deepseek-ai/dsh-client-ui-slots'
+import type { SessionMaybeProvideInfo } from '@deepseek-ai/dsh-client-ui-slots'
 import {
   createSlotRenderer, SessionProvider, SlotOwnershipError, StaleAuthorizationError,
   type RenderOpts, type SessionProvideInfo,
@@ -85,7 +86,9 @@ function makeHost() {
   const storeCache = new Map<StoredEntry, Map<string, StoreInstanceLike>>()
   const list = observable<{ ids: string[] }>({ ids: [] })
   const workspaces = observable<{ ids: string[] }>({ ids: [] })
-  const current = observable<string | undefined>(undefined)
+  const absentInfo: SessionMaybeProvideInfo = { sessionId: undefined, hooks: {}, props: {} }
+  const provide = observable<SessionMaybeProvideInfo>(absentInfo)
+  let currentId: string | undefined
   const infos = new Map<string, SessionProvideInfo>()
 
   const bump = (key: string) => {
@@ -123,10 +126,7 @@ function makeHost() {
     },
     sessions: {
       list,
-      current,
-      provideInfo: id => infos.get(id),
-      maybeProvideInfo: id => (id === undefined ? undefined : infos.get(id))
-        ?? { sessionId: undefined, hooks: {}, props: {} },
+      provide,
     },
     workspaces: { list: workspaces },
   }
@@ -134,7 +134,14 @@ function makeHost() {
     host,
     list,
     workspaces,
-    current,
+    // Same driver surface as the old current cell: set(id) publishes the
+    // resolved bundle (or the absent projection) through the provide source.
+    current: {
+      set: (id: string | undefined) => {
+        currentId = id
+        provide.set((id === undefined ? undefined : infos.get(id)) ?? absentInfo)
+      },
+    },
     declare: (key: string, spec: DeclaredSpec) => { specs.set(key, spec); bump(key) },
     add: (key: string, partial: Omit<StoredEntry, 'options'> & { options?: StoredEntry['options'] }) => {
       const entry = entryOf(partial)
@@ -161,6 +168,7 @@ function makeHost() {
         props: {},
       }
       infos.set(id, info)
+      if (currentId === id) provide.set(info)
       return info
     },
   }