fix(islands): ignore inactive session messages

[peter-gy]

Follow up to #10207.

Drops kernel messages after their session yields ownership of the shared islands frontend.

Retained navigation reuses one frontend store across app sessions. Messages already in transit from the outgoing session could otherwise apply cell or UI updates to the incoming document when cell IDs overlap.

The bridge now tracks accepted session generations. Replacement and teardown revoke the outgoing generation before their asynchronous worker calls begin. A failed replacement restores ownership to the previous session. Multi-app documents retain every generation started for the current document.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Jul 25, 2026 6:41pm

[cubic-dev-ai]

All reported issues were addressed across 2 files

Architecture diagram

sequenceDiagram
    participant Worker as Pyodide Worker
    participant Bridge as IslandsPyodideBridge
    participant Store as Jotai Store
    participant UI as React UI Components
    
    Note over Worker,UI: Active session flow
    
    Bridge->>Bridge: startSession(request)
    Bridge->>Bridge: activeSessionGenerations.add(generation)
    Bridge->>Store: Set islandsPendingInitialRunsAtom
    Bridge->>Worker: proxy.request.startSession(request)
    Worker-->>Bridge: session ready
    
    loop Kernel messages
        Worker->>Bridge: "kernelMessage" event
        alt activeSessionGenerations.has(sessionGeneration)
            Bridge->>Bridge: Forward to messageConsumer
            Bridge->>UI: Apply cell/UI updates
        else Session inactive
            Bridge->>Bridge: Ignore message
        end
    end
    
    Note over Worker,UI: Session replacement flow
    
    Bridge->>Bridge: initializeApps()
    Bridge->>Bridge: activeSessionGenerations.clear()
    Bridge->>Bridge: nextSessionGeneration++
    Bridge->>Bridge: startSession(request)
    Bridge->>Bridge: activeSessionGenerations.add(generation)
    
    alt Replaces previous single app
        Bridge->>Store: Reset notebookAtom
    end
    
    Bridge->>Worker: proxy.request.startSession(request)
    
    Note over Worker,Bridge: Asynchronous worker call in flight
    Worker->>Bridge: "kernelMessage" (old generation)
    alt Generation not in activeSessionGenerations
        Bridge->>Bridge: Drop message
    end
    
    Worker-->>Bridge: session ready
    Bridge->>Bridge: sessionReady.resolve()
    
    Note over Worker,UI: Session stop flow
    
    UI->>Bridge: stopSession()
    Bridge->>Bridge: activeSessionGenerations.delete(generation)
    Bridge->>Worker: proxy.request.stopSession(request)
    
    Note over Worker,Bridge: Asynchronous worker call in flight
    Worker->>Bridge: "kernelMessage" (stopped generation)
    alt Generation not in activeSessionGenerations
        Bridge->>Bridge: Ignore message
    end
    
    alt Error during session start
        Bridge->>Bridge: activeSessionGenerations.delete(generation)
        Bridge->>Bridge: Revert session to previous
        Bridge->>Store: Restore previous state
    end

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