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

packages/client/web-react/src/session-provider.tsx

@@ -83,6 +83,39 @@ function useAbsentSnapshot<S>(_selector: (snapshot: never) => S, _equal?: (a: S,
   return undefined
 }
 
+/**
+ * The useProjection framework seat (session-projection RFC), one bound
+ * function per provide bundle (cached by info identity — components may hold
+ * it across renders). Key-addressed: the key resolves a per-session cell
+ * source, whose bound selector hook comes from the same per-source cache as
+ * every other kit hook, so exactly one uSES subscription runs per call and
+ * the subscribe reference stays stable while the cell lives. An unresolved
+ * key (no cell, no session, plugin unloaded) reads `undefined` — capability
+ * absence — through the absent source, keeping the hook order constant.
+ */
+export function projectionHook(info: SessionMaybeProvideInfo): (
+  key: string, selector?: (value: unknown) => unknown, eq?: (a: unknown, b: unknown) => boolean
+) => unknown {
+  let hook = projectionHookCache.get(info)
+  if (hook === undefined) {
+    hook = (key, selector, eq) => {
+      const cell = info.projections?.cellOf(key)
+      // The absent branch binds the shared absent source so the caller's
+      // selector still runs over `undefined` (absence flows through the
+      // selector) and the uSES call count stays constant across resolution.
+      const useCell = observableHook(cell ?? absentSource)
+      // Whole values are frozen event/wire data (identical reference between
+      // events), so the identity selector needs no equality function.
+      return useCell(selector ?? (value => value), eq)
+    }
+    projectionHookCache.set(info, hook)
+  }
+  return hook
+}
+const projectionHookCache = new WeakMap<SessionMaybeProvideInfo, (
+  key: string, selector?: (value: unknown) => unknown, eq?: (a: unknown, b: unknown) => boolean
+) => unknown>()
+
 /**
  * Root-level binding provider. It follows current selection without a key, so
  * session-maybe entries retain their React identity while the context value