feat(narrative): draft safety guard, scene loader race protection and autosave

[lark-x]

Summary

• Implemented \useSceneNavigationGuard\ for intercepting scene changes / route navigation / beforeunload on dirty scene drafts.
• Added unsaved changes confirmation modal with 'Save & Proceed', 'Discard', and 'Cancel' options.
• Implemented \useNarrativeSceneLoader\ with \loadGeneration\ token and \AbortController\ cancellation.
• Implemented \useNarrativeAutosave\ with 1000ms debounce and conflict detection.
• Added full unit test suite in \scene-draft-safety.test.ts.