Kotlin Multiplatform

This is specifically built for the Amethyst KMP project but the patterns are solid for any Kotlin Multiplatform codebase. It walks through the abstraction decision tree (should this be commonMain, jvmAndroid, or expect/actual?) with real examples from crypto, JSON parsing, and navigation. The jvmAndroid pattern is interesting: it's a custom source set that shares JVM libraries like Jackson between Android and Desktop, which saves duplication but won't work for iOS or web. Most useful when you're staring at code wondering if it belongs in the common module or needs platform splits. The decision tree alone is worth keeping around.