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/stale-authorization.spec.tsx

@@ -21,6 +21,7 @@ function makeHost() {
   const versions = new Map<string, number>()
   const subs = new Map<string, Set<() => void>>()
   const live = new Set<StoredEntry>()
+  const absentInfo = { sessionId: undefined, hooks: {}, props: {} }
   const bump = (key: string) => {
     versions.set(key, (versions.get(key) ?? 0) + 1)
     for (const fn of [...(subs.get(key) ?? [])]) fn()
@@ -39,9 +40,7 @@ function makeHost() {
     storeOf: () => undefined,
     sessions: {
       list: { getSnapshot: () => ({}), subscribe: () => () => {} },
-      current: { getSnapshot: () => undefined, subscribe: () => () => {} },
-      provideInfo: () => undefined,
-      maybeProvideInfo: () => ({ sessionId: undefined, hooks: {}, props: {} }),
+      provide: { getSnapshot: () => absentInfo, subscribe: () => () => {} },
     },
     workspaces: {
       list: { getSnapshot: () => ({}), subscribe: () => () => {} },