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-conversation/tests/chat-view.spec.tsx

@@ -105,6 +105,7 @@ function makeHarness(init?: Partial<ConversationSnapshot>) {
     useSession: bindSnapshotSelector(source),
     useSessions: emptySessions(),
     useWorkspaces: emptyWorkspaces(),
+    useProjection: (() => undefined),
     useInput: (() => { throw new Error('unused') }),
     inputActions: { setDraft: () => {}, submit: () => {} },
     useStore: bindSnapshotSelector(chat),