Stackflow transition breaks when activity suspends longer than transitionDuration

Currently, Stackflow activities' enter transition is only expressed with enter-active and enter-done. However, this doesn't work well with frameworks that do concurrent rendering because the core state update happens regardless of whether activities render synchronously or suspend during render. This results in something like this:

sequenceDiagram
    participant Core as @stackflow/core
    participant Integration as Integration (@stackflow/react)
    participant View
    View->>Core: push()
    activate Core
    note left of Core: Transition
    Core->>Integration: notify on subscribe() listener
    Integration->>Core: getStack()
    Core->>Integration: State with `enter-active`
    Integration->>View: Render new activity with `enter-active`
    activate View
    note right of View: Suspend
    Core->>Integration: notify on subscribe() listener
    deactivate Core
    Integration->>Core: getStack()
    Core->>Integration: State with `enter-done`
    Integration->>View: Rerender with `enter-done`
    deactivate View
    note right of View: Initial render with `enter-done`

As described above, if the view suspends longer than the core transition, the initial render of the view is done with enter-done state, resulting in broken CSS transitions.

To mitigate this, the core transition should be started after confirming that the initial view was rendered. These are the required parts to implement the feature:

• Add a new transition state to express activities that didn't complete the initial render. (enter-pending?)
• Add a new action that notifies the core after completing the initial render. (no idea about naming 🥲)

I'm willing to implement the feature after the direction is fixed.

Related: #434

Start by tracing the transition state and subscription flow between @stackflow/core and @stackflow/react, using the sequence in this issue and related issue #434 as context. Define how the pending state and post-render action should interact with suspended activities. Done means the initial render cannot skip the enter transition when suspension outlasts transitionDuration, with the behavior covered by the relevant existing tests.