feat(gui): useProjection — the fifth framework hook seat through the standard kit

React half of the session-projection client base: the renderer contract gains
an open-key projections face on SessionMaybeProvideInfo (cellOf(key), distinct
from the static hooks roster), web-react mints projectionHook (per-bundle cache;
per-cell uSES binding via the shared observableHook cache; unresolved keys read
undefined through the absent source so hook order stays constant), standardKit
delivers kit.useProjection, and the runtime merges UseProjection into
SessionStandardProps/SessionMaybeStandardProps (overloads mirror useSession).
3 jsdom specs (kit delivery + live re-render, selector over undefined, faceless
bundle = all absent); existing direct-prop-feed specs gain the one-line stub the
new required seat mandates.

packages/client/web-react/src/scoped-slots.tsx

@@ -10,7 +10,7 @@ import {
 } from '@deepseek-ai/dsh-client-ui-slots'
 import {
   HostContext, SessionMaybeProvider, SessionProvider, SlotAssemblyError, maybeObservableHook,
-  observableHook, useHost, useSessionMaybeProvideInfo,
+  observableHook, projectionHook, useHost, useSessionMaybeProvideInfo,
 } from './session-provider.tsx'
 
 type InjectedProps = Record<string, unknown>
@@ -219,6 +219,9 @@ function standardKit(
     }
     Object.assign(kit, info.props)
     kit['sessionId'] = info.sessionId
+    // The useProjection seat (fifth framework hook): key-addressed cell
+    // reader, bound per provide bundle (cached by info identity).
+    kit['useProjection'] = projectionHook(info)
   }
   const store = scope === 'session-maybe' && info?.sessionId === undefined
     ? undefined