The sessionStorage seed set loading=false whenever any cache existed, which
defeats App.tsx's load-bearing pluginsLoading gate: with a cached manifest
that declares tab.override === "/chat", the persistent ChatPage host must
NOT mount before plugins resolve, or it spawns a PTY and gets yanked when
the override plugin takes over the route.
Seed loading=false from the cache only when no cached manifest overrides
/chat (canSeedLoadedFromCache); manifests are still seeded either way so
plugin routes register synchronously on refresh. Adds focused tests for
the gate, including the /chat-override case.
c0c45ab68a