T3: migration.exe — New Native Executable

[Varaniya201]

1. Implement idempotent JRE backup: jre/ → jre11/ (skip if already exists)
2. Implement idempotent JRE promotion: jre21_temp/ → jre/ (unzip from .artifacts/jre21.zip if jre21_temp/ missing)
3. Implement lib/ cleanup and _launcher.jar + run.bat restore from .artifacts/
4. Implement failure detection and auto-invocation of rollback.exe
5. Implement operator dialog on success: "JRE migration complete. Please start the application using run.bat."