added readme's

sandbox/README.md

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+# sandbox
+
+A playground for working out the implementation and architecture of new engine features. It can include the engine's internal backend headers (`engine/src`) as well as the public API, so you can use classes such as `Device`, `RenderContext`, `RenderGraph` and `SceneRenderer` directly.
+
+Prototype a feature here first. Once its design settles, move it into the engine and expose it through the public API, which [testbed](../testbed/README.md) exercises.
+
+Any `.cpp` file in this directory is compiled in automatically.
+
+## Running
+
+```sh
+dev sandbox [args...]
+```
+
+This builds the `debug` preset and runs `build/debug/bin/sandbox`. Shaders are loaded from `shaders/` (`SANDBOX_SHADER_DIR`).

testbed/README.md

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+# testbed
+
+Exercises the engine's frontend API. It includes only the public headers in `engine/include/sophia/`, so it uses the engine the way a game would.
+
+Use it to check that public API changes work and feel right from the caller's side. It cannot include engine internals (`engine/src`). If something needs them, it belongs in [sandbox](../sandbox/README.md) instead.
+
+## Running
+
+```sh
+dev testbed [args...]
+```
+
+This builds the `debug` preset and runs `build/debug/bin/testbed`. Shaders are loaded from `shaders/` (`TESTBED_SHADER_DIR`).