Panel Fixed Effects Filtered and Variance Decomposition Estimation // Implements fixed effects estimators for time- invariant variables in panel data models. Provides three estimation methods: FEVD (Fixed Effects Vector Decomposition) from Plumper and Troeger (2007) doi:10.1093/pan/mpm002, and FEF (Fixed Effects Filtered) and FEF-IV (instrumental variables variant) from Pesaran and Zhou (2018) doi:10.1080/07474938.2016.1222225. All methods use the correct Pesaran-Zhou variance estimators that account for generated regressor uncertainty, avoiding the size distortions documented in the literature.