Make the public `instant()` navigation-testing helper resilient when the `next-instant-navigation-testing` cookie survives a previous scope. A stale cookie already present when a new scope starts must not be mistaken for an active scope: the callback should run with instant navigation enabled, and normal navigation should resume after it exits.

Preserve real scope-conflict behavior. Overlapping or nested `instant()` calls that share one browser context must still be rejected, while concurrent scopes in separate browser contexts or browsers must operate independently. Scope cleanup must leave the testing cookie gone even when an MPA load asynchronously restores it around release, including when the callback fails, without disturbing unrelated application cookies. Later `instant()` calls in the reused context must continue to work.