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/ui-slots/src/renderer.ts

@@ -44,6 +44,14 @@ export interface SessionMaybeProvideInfo {
   hooks: Record<string, HostObservable<unknown> | undefined>
   /** Static plain-member roster; values are undefined with the session. */
   props: Record<string, unknown>
+  /**
+   * Key-addressed projection-cell sources (the useProjection framework seat,
+   * session-projection RFC). Unlike `hooks`, the key space is open — cells
+   * come and go with domain plugins — so the render side binds per resolved
+   * cell instead of per static roster member. Absent with the session; an
+   * unresolved key uniformly reads as capability absent.
+   */
+  projections?: { cellOf(key: string): HostObservable<unknown> | undefined } | undefined
 }
 
 /** Definite per-session standard props resolved for strict session slots. */