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/gate-branch-tails.spec.tsx

@@ -76,6 +76,7 @@ describe('render branch tails', () => {
         useSession={bindSnapshotSelector({ getSnapshot: () => snap, subscribe: () => () => {} })}
         useSessions={bindSnapshotSelector(emptyList)}
         useWorkspaces={bindSnapshotSelector(emptyWorkspaces)}
+        useProjection={(() => undefined)}
         useInput={(() => { throw new Error('unused') })}
         inputActions={{ setDraft: () => {}, submit: () => {} }}
         useStore={bindSnapshotSelector(chat)}
@@ -111,6 +112,7 @@ describe('render branch tails', () => {
         useSession={bindSnapshotSelector({ getSnapshot: () => snap, subscribe: () => () => {} })}
         useSessions={bindSnapshotSelector(emptyList)}
         useWorkspaces={bindSnapshotSelector(emptyWorkspaces)}
+        useProjection={(() => undefined)}
         useInput={(() => { throw new Error('unused') })}
         inputActions={{ setDraft: () => {}, submit: () => {} }}
         useStore={bindSnapshotSelector(chat)}