docs: add a Role in the framework section to every module README

Each module README now says where the module sits in the dependency
graph, why it is its own module, and lists use cases for application
developers.

modules/compass/README.md

@@ -10,6 +10,16 @@ Layered YAML configuration with per-environment directories, `SUMMER_` environme
 
 `compass` is the SummerCMS counterpart of WinterCMS's `config/*.php` files, per-environment config directories, `.env` support and `Config::get('app.name')`. It merges several layers into one tree built on koanf, and every key is read with a dot path such as `app.name`. Plugin defaults live under the bare plugin ID (`acme.blog.posts_per_page`) instead of WinterCMS's `acme.blog::posts_per_page`. The generated `main` of an application calls `compass.Load("config")` and hands the result to [backpack](../backpack/README.md); [party](../party/README.md) merges plugin defaults into it during activation.
 
+## Role in the framework
+
+A dependency-free foundation module that `backpack` holds and most service modules read from. It is separate so configuration loading can run before the application exists, and so any package can read settings without depending on the container.
+
+Use cases:
+
+- Keep shared settings in a base file and override them per environment.
+- Override any key from the environment, for example in containers and CI.
+- Ship plugin defaults embedded in the plugin, so the plugin works with no host configuration.
+
 ## Features
 
 - A fixed layer order, lowest to highest precedence: