nowcast 0.1.0

• Initial CRAN release.
• Bridge equation nowcasting via nc_bridge() with optional AR terms.
• Mixed-frequency alignment via nc_align() with ragged-edge diagnostics.
• Temporal aggregation (nc_aggregate()) and stationarity transforms (nc_transform()).
• Pseudo-real-time backtesting via nc_backtest() with expanding or rolling windows.
• Diebold-Mariano test (nc_dm_test()) with Harvey-Leybourne-Newbold (1997) finite-sample correction.
• Evaluation metrics (nc_evaluate()): RMSE, MAE, and bias.
• S3 methods: print(), summary(), plot(), and predict() for all core objects.