Explain the KMP source-set hierarchy and how intermediate source sets like appleMain enable code sharing.

## Source sets as a dependency tree A **source set** is a named group of Kotlin files with its own dependencies and visible API surface. KMP arranges them into a **hierarchy (a tree of `dependsOn` edges)**: ``` commonMain ├── jvmMain (Android/JVM) ├── jsMain └── nativeMain └── appleMain ├── iosMain │ ├── iosArm64Main │ └── iosSimulatorArm64Main └── macosMain ``` - **`commonMain`** — the root; visible to every target; can use only multiplatform APIs. - **Leaf sets** — e.g. `iosArm64Main`, one per concrete target. - **Intermediate source sets** — e.g. `nativeMain`, `appleMain`, `iosMain` — group targets that share capabilities. ## Why intermediate sets matter Without them, code that works on all Apple targets would have to be copy-pasted into `iosArm64Main`, `iosSimulatorArm64Main`, `macosArm64Main`, etc. An intermediate set like **`appleMain`** is compiled once and shared by all Apple descendants, and it can see the **Foundation/Darwin** APIs common to them. Similarly `nativeMain` can use POSIX-style APIs common to all native targets. ## API visibility follows the tree A source set sees: - the **intersection** of APIs available to all of its descendant targets, plus - everything declared in its **ancestor** source sets. So `appleMain` sees `commonMain` + `nativeMain` + the Apple-common platform APIs, but **not** Android or JS APIs. ## expect/actual across the hierarchy An `expect` declared in `commonMain` can be `actual`-ized in an **intermediate** set (e.g. one `actual` in `appleMain` covering all Apple targets) instead of repeating it in each leaf — a big duplication win. ## The default hierarchy template Modern Kotlin auto-applies a **default hierarchy template** (you can call `applyDefaultHierarchyTemplate()` explicitly) that creates `nativeMain`, `appleMain`, `iosMain`, etc. based on the targets you declare. Before this, teams hand-wired `dependsOn` edges. Each `*Main` set has a parallel `*Test` set (`commonTest`, `appleTest`, …) following the same tree. ```kotlin kotlin { androidTarget() iosArm64(); iosSimulatorArm64() // applyDefaultHierarchyTemplate() // usually implicit sourceSets { commonMain.dependencies { /* shared libs */ } // appleMain is created for you and groups iOS targets } } ```