March 2026 Monthly Summary: Buck2 packaging stability improvements across Buck2 and Buck2-Prelude. Implemented a fix for a crash in fat_jar.py that occurred when combining --concat_jars with --meta_inf_directory by packaging the meta_inf_staging directory into a jar before handing it to the jar builder. The same fix was applied to buck2-prelude to ensure consistent behavior across repositories. These changes eliminate the IllegalStateException raised by ConcatJarBuilder when it encounters non-jar entries and reduce CI/build flakiness related to fat-jar packaging. Impact: More reliable artifact packaging, fewer build-time crashes in multi-jar scenarios, and smoother downstream consumption of Buck artifacts. The changes align packaging semantics between core Buck2 and the Prelude module, improving maintainability and reducing surprises for users relying on concatenated jars. Review and collaboration: Changes reviewed by NavidQar (Differential Revision: D95985374; fbshipit-source-id: 40654760b6f9abf926b81c7fbcb3d21e1ac5eda1).