Remove startThumbnail and startActors from Stage type

[bengotow]

This PR removes the startThumbnail and startActors properties from the Stage type and all related functionality for saving and restoring initial game state.

Summary

The initial game state save/restore feature has been completely removed from the codebase. This includes the data structures that stored thumbnail images and actor states at the beginning of a stage.

Key Changes

• Removed startThumbnail and startActors properties from the Stage type definition in both frontend and headless type files
• Deleted saveInitialGameState() and restoreInitialGameState() action creators from stage-actions.ts
• Removed corresponding action type constants SAVE_INITIAL_GAME_STATE and RESTORE_INITIAL_GAME_STATE
• Removed reducer cases handling these actions in stage-reducer.ts
• Removed the restoreInitialGameState dispatch call from root-player.tsx
• Cleaned up initial state definitions in initial-state-stage.ts and initial-state-stage-tutorial.ts by removing the large base64 thumbnail data and actor objects
• Updated test fixtures and data migration tests to remove references to these properties
• Removed the RESTORE_INITIAL_GAME_STATE case from world-reducer.ts

Implementation Details

• The tutorial stage definition had 674 lines of actor data that have been removed
• All references to initial game state restoration have been eliminated from the world reducer
• The changes maintain backward compatibility by simply removing unused functionality rather than adding deprecation warnings

https://claude.ai/code/session_01Tz6UrBPXv4y39VUsc3aAKj