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/runtime/src/client/index.ts

@@ -7,6 +7,7 @@ import { SessionsService } from './sessions/service.ts'
 import type { SessionListState } from './sessions/service.ts'
 import { WorkspacesService } from './workspaces/service.ts'
 import type { ConversationSnapshot, RunningToolCall, ToolResultNode } from './sessions/conversation.ts'
+import type { UseProjection } from './sessions/projection-cell.ts'
 
 export { SlotsService } from './slots.ts'
 export type { RootOwnerProps } from './slots.ts'
@@ -34,6 +35,12 @@ export type {
 } from './sessions/conversation.ts'
 export { PendingWait } from './sessions/pending.ts'
 export type { PendingInteraction, PendingKind, PendingPayloads } from './sessions/pending.ts'
+// Projection cells (session-projection RFC): domain plugins register cells at
+// scope materialization via `binding.session.projections.register(spec)`.
+export type {
+  ProjectionCell, ProjectionCellSet, ProjectionCellSpec, ProjectionSchemaLike, ProjectionsBaseline,
+  SessionProjectionMap, UseProjection,
+} from './sessions/projection-cell.ts'
 export type { SessionId } from '@deepseek-ai/dsh-client-connection/client'
 
 /** Client-side Cordis context after declaration merging. */
@@ -59,12 +66,16 @@ declare module '@deepseek-ai/dsh-client-ui-slots' {
     useSession: SnapshotSelectorHook<ConversationSnapshot>
     /** The framework-resolved session id (owners never pass it). */
     sessionId: SessionId
+    /** The fifth framework hook seat: key-addressed projection reader (undefined = capability absent). */
+    useProjection: UseProjection
   }
   /** Standard kit for slots that remain mounted while current session changes. */
   interface SessionMaybeStandardProps {
     useSession: MaybeSnapshotSelectorHook<ConversationSnapshot>
     /** Current session id; absent in the no-session state. */
     sessionId: SessionId | undefined
+    /** Key-addressed projection reader; every key reads absent while no session is current. */
+    useProjection: UseProjection
   }
   /** Props injected into every global slot component. */
   interface GlobalStandardProps {