Add Java scaffold (Maven multi-module, JUnit 5 + AssertJ, mise toolchain)

java/README.md

@@ -0,0 +1,23 @@
+# Show, Don't Tell — Java exercise workspace
+
+Java track for the "Show, Don't Tell — Using Examples with Coding Agents" exercise (Java 21 / Maven / JUnit 5 / AssertJ).
+
+## Prerequisites
+
+- Java 21 (Temurin) and Maven, versions pinned in `mise.toml`
+- If you use [mise](https://mise.jdx.dev): `mise install` installs both
+
+## Build and run the tests
+
+```sh
+mvn test
+```
+
+## Layout
+
+| Module | Contents |
+| --- | --- |
+| `order-domain`, `order-tests` | Order domain with discount behaviour, and its tests (Round 1) |
+| `order-book-domain`, `order-book-tests` | OrderBook state machine, and its integration tests (Round 2) |
+
+The exercise starts in its START state: all tests are green, the refactoring participants should do is NOT applied. The custom matcher example lives in `order-tests` (`OrderStateAssertion`).